Northern-centric much?
Here's some more closed or freight-only southern lines that would be useful:
Totton to Fawley
Brockenhurst to Wimborne (via Ringwood)
Bordon spur
Pewsey to Holt Junction (via Devizes) as an extension of Bedwyn line services
Portishead branch as an extension of Avonmouth services
Henbury loop
Stansted Airport to Braintree
Smallbrook Junction to Newport and/or Cowes
(ambitious) Andover to Swindon (via Marlborough)
Alton to Winchester
Some comments:
Totton to Fawley:
Fawley, population about 14500
Marchwood 5500
Possibly not quite large enough to support a passenger service.
Pewsey - Devizes - Holt.
Devizes population 11300. Any service would ideally need to run through to Bath & Bristol, but with no other significant population centres on the route, I think Devizes is too small to hope for a restored railway service.
Portishead branch . With a population of about 22000, Portishead branch has a fairly good case. Problem for commuters is that Bristol Temple Meads is not ideally sited for the city centre shopping & business areas. Best solution might be a tramway running via city centre.
Stansted Airport - Braintree. Case depends on how much traffic is there from East Anglia to the Airport - and a problem is that the junction at Witham is in the wrong direction for through trains from Norwich / Ipswich / Colchester to the Braintree line.
Andover to Swindon.
Not much chance, with Marlborough (8000) and Ludgershall (3700) being the only significant places on a longish route.
Cowes - Newport (IOW) - Ryde. A bit silly to close it in my opinion, but probably not enough population to get money for reinstatement. (Cowes 9000 or about 15000 dependent on source of figures*, Newport 24000, Ryde 30000)
(* either
http://www.wightonline.co.uk/islandinfo/population.html
or wikipedia)
Ventnor IOW (5900) - also silly to close this section of line, but probably now considered too small to support reopening.
Alton to Winchester. Again, a bit silly to close the southern section, but apart from cost, I read somewhere that line capacity at Winchester Junction would be a problem if it reopened.
Bordon (Hants). Population about 15000. Maybe one for the future if the population increases significantly.
Henbury (Bristol). Population about 9500. Are you thinking of a Bristol - Avonmouth - Henbury - Bristol loop service ? With another 12000 at Avonmouth, there may not be enough people to support such a service - another possible candidate for conversion to a tramway (if any freight services can be accommodated) ?