Might I suggest... Leighton Buzzard.
Why? Well, I used to live there, and there are fast trains into London Euston - 30 mins or something I think. But instead of paying thousands a month to rent, you pay hundreds.
Honestly not LB in particular, but unless you want to live the London life, commuting in on the really very good train system is the way to go.
Unless there are multiple people all working in the city, at which point the train passes might be more expensive.
Also, there are lots of jobs in the towns and cities that surround London - Hemel Hempstead, Watford, St. Albans, etc, etc, etc.
I mean, there are so many cities... the South East is busy (Essex, Kent, Herts, etc, etc, etc). Get up to the Midlands and there are still lots of things to see and do but it's less expensive to live. Move further north and it's cheaper again.
South West is more expensive to live and less going on in terms of jobs than the South East, AFAIK.
Wales... Hmm, well, Cardiff and the surrounds are busy, but mid and north Wales are much more rural. The north west of England is also less busy.
But yeah, possible to live frugally in most places. If you're going to work in London you'll get a London weighting to compensate for the silly prices.