Good topic, I'll try to go through the 2-axle wagon types in alphabetical order that are still in service.
I'll ignore wagons in the international series used in the UK, which would be some of the STVA car carrier types already mentioned, but also (perhaps surprisingly) the IFA container wagons used on the Transfesa flow from Dagenham / Barking via HS2 to Spain - in this photo the angle of the sun highlights the wheel arrangement if you zoom in:
https://www.flickr.com/photos/papercliponawire/14549742566/
I'll include infrastructure wagons - there are surprisingly few left in active service after recent deliveries of new Wascosa box wagons and modular FEAs.
I've probably missed one or two obscure types but this should be reasonably comprehensive. Headcodes are the typical loaded and empty workings where known, actual days run will vary...
CDA - China clay - Goonbarrow to Fowey Docks (6G09/ 6G05) and Treviscoe to Fowey Docks (6P24 / 6P07)
MEA - Coal - Redcar to Oxwellmains Cement Works - 6Z69 / 6Z70 (this set was formerly used on Redcar to Scunthorpe Ground Granulated Blast Furnace Slag traffic )
MEA - Gypsum - Tees Dock to Newbigin - 6M97 / 6E97 (this set was formerly used on Cardiff to Rotherham processed slag "steelphalt" traffic)
I'm not aware of MEAs ever being in regular use on scrap traffic (lower payload and less robust body strengthening compared to SSAs for example). Their use on the Cardiff - Rotherham circuit was for processed slag "steelphalt" which is used as an aggregate for road surfacing, they went empty back to Cardiff while the scrap was loaded in trains of MBAs (using the same wagons for both scrap and slag would likely have caused contamination issues).
MFA, MHA, MPA "Coalfish" - infrastructure open wagons. Appear to be less than 200 in traffic and those do not seem to be very active. I think only one possession train ran with coalfish last weekend (6Y60 Doncaster - Totley tunnel)
OCA - "Bass" open wagon with steel doors for general engineers materials. Seem to be about two dozen left in traffic although not active recently
PCA - Alcan alumina, North Blyth to Fort William - 6S45 / 6E45
PCA - Cement - Clitheroe to Avonmouth - 6V41 / 6M42
PCA - Cement - Hope to Walsall - 6G65 / 6H62
PCA - Cement - Hope to Dewsbury - 6E91 / 6M89
PCA - Cement - Ketton to St Pancras - 6M99 / 6F93
PNA - Plasmor blocks - Heck to Biggleswade and Bow - 6H86, 6L69 / 4E25
PFA (ex Cawoods) at Sellafield
Sellafield to Drigg low level waste -
https://www.flickr.com/photos/75784477@N08/52333276700/
Winfrith to Drigg low level waste -
https://www.flickr.com/photos/64657021@N07/52943838810/
Sellafield to Barrow (acting as barrier wagons on special flask moves) -
https://www.flickr.com/photos/75784477@N08/41298946220/
(These are some recent uses of the PFAs - I think there have been some other flows of low level waste or use as barriers on special flask movements using these wagons but I'm not too sure of currently active flows)
RRA - Runner wagons (converted from various types). Less than ten in service, used for overhanging loads on the three FZA "Super Salmon" wagons - see this photo for an example of their occasional use on infrastructure trains (click right arrow for a couple more photos with informative captions) -
https://www.flickr.com/photos/36034969@N08/50661572753/in/photolist-2kbMNyi-2iiQWZV-2gQGvDU-2gQGuha/
This 150 page document on infrastructure loading patterns for various wagons including FZAs may also be of interest:
https://safety.networkrail.co.uk/wp-content/uploads/2019/05/Loading-Pattern-Combined.pdf
SPA - none in traffic. I think the last ones used on steel traffic were on the Deepcar to Aldwarke flow, and the ones used on infrastructure traffic (as "Pike" for conveying sleepers) are now stored.
SSA - scrap - Trostre to Port Talbot (weekly train) - usually 6B03 Friday / 6B01 Sunday
SSA - scrap - Corby to Port Talbot. A handful of wagons are often attached to the steel coil train 6M94 / 6V92 - interesting photo taken at Corby:
https://www.flickr.com/photos/andy_hoare/52031960402/
VGA - Wire Rod in Coil - Cardiff Tidal to Burton - 6M50 / 6V50
VGA & VKA - MoD traffic (various Bicester / Kineton / Longtown / Marchwood e.t.c.)
TUA - Ethanol and Methanol - Immingham Tank Farm West to Lindsey refinery. Local trip, I presume the contents are used to blend into fuels like E5 or E10 which contain a percentage of bioethanol.
Immingham West tank farm is better known as "Simon Storage" but is now "Inter Terminals Immingham West"
Runs as required, mainly (or possibly entirely) within the non-public port area, so is rarely photographed - but I did find this picture:
https://www.flickr.com/photos/dbs60100/40696436583
This is a typical schedule for these trips -
https://www.realtimetrains.co.uk/service/gb-nr:80467/2023-07-25/detailed (if that link is out of date, it shows a VSTP schedule for a 12:30 departure from Immingham Storage W Terminal to Lindsey Oil Refinery Colas, with intermediate timings at Immingham West Jn and Humber Road Jn, running about an hour early).
For this traffic there's a fleet of two-dozen TUAs in the NACO 74xxx and VTG 701xx number range (similar to TTAs but with a larger capacity and heavier axle weight). I remember the NACO TUA tanks being used years ago on a flow out of Harwich (Carless Refinery) to Aberdeen with "mud oil" for drilling operations -
https://www.flickr.com/photos/37001/6447519667
I think the last PGAs in service were former "Redland Aggregates" wagons (ordinary hoppers, not the ones with conveyors underneath) which migrated to Cliffe (near Hoo Junction, Kent) and were used on aggregate trains to Tolworth and Purley in grotty mixed rakes which also included the orange bogie hoppers that used to have RMC branding and worked out of Peak Forest. See this photo from 2018 for a good example, there are several wagons pictured if you click left and right -
https://www.flickr.com/photos/queenfanjohn/39269255920/in/photolist
I think 2018 was probably the last year any PGAs were in service. I think nearly all are scrapped (were any preserved?) and possibly a couple of them might still be languishing in the depths of Wembley yard...
I remember the smart ECC blue PGA hoppers, when they had the ECC branding I think this set used to run as a block train on a circuit from Croft (near Leicester) to either Bow or Bishops Stortford, as required. Pictured here:
https://www.flickr.com/photos/64518788@N05/40420424755
I think your best bet for PCAs are the services from Hope to Walsall and Dewsbury, as it looks like the 2-axle wagons have been concentrated on the shorter distance flows. Theale and Dagenham trains usually run with bogie cement wagons, although a while back I think PCAs were fairly common on the Dagenham trains. There were also trains of flyash from Drax to Hope using PCAs, but I think these stopped running last year.