This has been a fascinating thread, and I thank all the contributors very much. I suspect that one of the reasons it has generated so many ideas is that until recently, buses were in general much more built to the exact specifications of the operator and not off-the-shelf. Take as an example my own Bristol RE, built with Marshall body: Bristol RE buses with Marshall bodies were supplied to North Western, Southdown, Aldershot & District, East Midland, Ribble, South Wales and Western Welsh - some 250 buses for seven customers. Yet some had Gardner engines, some Leyland, RESL or RELL, making a difference of 4 feet in length; and don't get me started on seat moquette, seat layout, height of the front grille and so on. So very much not bespoke, but every operator specified things that only it required.
Perhaps there are several definitions of 'bespoke'. Here's my attempt, with example:
1. A vehicle type that's obviously different from the norm is specified by one customer alone, and delivered only to that customer. Examples would be BMMO's postwar output, also the very short Walsall Fleetlines (but see option 3 below), or London FRM1.
2. A vehicle that is offered for general sale but which performs so badly that only one operator buys it. Examples would be the SHMD Atkinson double decker, or the Marshall double deck bodies bought by Leicester. I'd also put the dual-door Edinburgh Lynxes here, as it was a generally-available option that, to the best of my knowledge, only Lothian availed themselves of. You could also put prototypes in this category where only one or two were made before the project ended.
3. Vehicles that were originally built to the specifications of one operator, but a couple of others like the look of it and also buy it. Examples would be the Routemasters bought by Northern; the Mancunian, sold to neighbour/rival Salford as the 'Salcunian'; the later Walsall Fleetline design, which few people appreciate that the Stalybridge Joint Board also bought; or the 1960s Liverpool body design, also bought by Bury and Bolton Corporations. You could also plead that the GM Standard Northern Counties design fits into this category as it was taken by several independents such as Delaine.
4. A vehicle that isn't exactly bespoke in being designed from the floor up for the requirements of one operator, but with more structural (usually body) modification than the normal run-of-the-mill specification changes like the Bristol REs mentioned above. Examples would be Glenton Tours' centre entrances long after everyone else had gone to front entrances; East Yorkshire's Beverley Bar buses; or Tyneside's nearside staircases. Were South Yorkshire unique in specifying Rolls Royce engines for many of their Dennis Dominators?
5. Modifications that aren't truly bespoke but are sufficiently unusual or distinctive that they're regarded as 'signature' choices by the operator. The difference between 4 and 5 is subjective but I'd put Salford's strange extended blind handle frames in this category, or the aforementioned Hants & Dorset ferry cut-aways. I'd also put later GM Standards in this category as I think they were the only ones to specify a particular automatic gearbox.
6. Unique combinations of chassis and body - the body is common, the chassis is common, but the combination is unique. For example there were seven Foden-NC buses completed but only one had an East Lancs body. Or the Albion Aberdonians supplied to Manchester with Seddon bodies (and nasty things they were, too).
It's a very interesting topic!
1. It doesn't get much more bespoke than this...
1965 BHA399C Midland Red BMMO D9 by
Andrew Farmer, on Flickr
2. The Atkinson PD746 wasn't designed for SHMD - they were the only people unlucky enough to buy one...
SHMD 70 UMA370 Boyle St 4-12-10 by
Mark Amis, on Flickr
3. Walsall weren't the only people to take short Fleetlines.
An unconventional bus by
Museum of Transport Greater Manchester archive, on Flickr
Liverpool to Bury in one body design...
Liverpool L501 501KD & Bury 116 REN116 Merseyside Transport Trust Open Day 14 July 2013 by
DMC1947, on Flickr
4. Nearside staircases were unique to Tyneside. Forward visibility from the lower deck wasn't great!
GBB516K Leyland Atlantean TWPTE 716 Pilgrim St Newcastle 22/4/1977 by
Geoff Allan, on Flickr
5. Only Salford specified the big frame for the blind handles so the conductor didn't have to climb. Does that make this bus bespoke?
Gloss amongst the grime by
Museum of Transport Greater Manchester archive, on Flickr
6. Very few Foden-NCs, and only one of them was East Lancs.
South Yorkshire PTE: 511 (SWG311S) in Leopold Street, Sheffield by
Mega Anorak, on Flickr