Rather then building new towns for commuters to the South East, I'd suggest a new town program in the North West and South West, with an industrial subsidy scheme (similar to the original new town programs) to relocate companies away from the South East.
North West:
Firstly, I'd propose a new town joining Rishton (which already has a station), Clayton-le-Moors and Great Harwood.
Land is cheap in the area, there is good motorway access, and you could link a tram system using the old Great Harwood loop alignment of the East Lancs line for the most part to serve the new developments with links into Blackburn and Burnley.
If the new town released additional transport-related funding, I'd electrify the east Lancs from the WCML to Colne and run the Preston - Colne stopper up to every 30 mins, if possible. This would link the new town into long distance services at Preston and services to Manchester/Leeds/Clitheroe/Blackpool at Blackburn/Burnley/Accrington.
Secondly, I'd select Acton Bridge in Cheshire as another potential site - the WCML station has good links to Liverpool and Birmingham, with stations at Cuddington and Greenbank close by towards Manchester and Chester.
Land is flat and not massively expensive in that area of Cheshire, and if you build towards Cuddington from Acton Bridge station, you're building away from the river Weaver and so avoid flooding issues in the area.
You also have the larger amenities a town of around 40 to 50k would need in Northwich or Runcorn, which are both reasonably nearby.
For the SW towns, I'd suggest St Germans as an initial development.
Good links into Plymouth/St Austell/Exeter by train, space to the west of the existing village to build on and establishing large industrial estates in the area would provide more much-needed jobs in a quite deprived county.
There's also potential to build a second station in the Liskeard direction, near Trerulefoot, to serve additional extensions if needed.
It is also a very desirable area to build in, with the picturesque Lynher valley to the east. Recreational infrastructure including hiking trails and parks could be developed along this area, as an eastern buffer to the large development.
The next place I'd consider would be Yeoford in Mid Devon. Regular services into Exeter on the Tarka line, and potential of more if small upgrades were made for the Okehampton service to be able to stop here.
Plenty of land is available in the area, it is relatively close to the A30, and being in the Yeo valley, land is reasonably flat.
The traditional agricultural economy in this part of Devon is currently struggling (partly due to the rising costs of farming), especially since some of the area supplements income via small-scale tourist enterprises.
As we've just seen for the last few years, the tourism and hospitality industry is not bulletproof, and the new town development could provide more secure employment for locals, as well as a nearby source of labour for the agricultural industry.
Again, there's potential for another station towards Okehampton/Barnstaple, if the new town was to build future extensions in the direction of Colebrooke/Penstone.