Hiya
St Albans is generally considered to be very nice indeed, quite alot like many towns just on the outskirts of outer London - very close to pop into the business of the city and far enough to go just 5 mins out and you're in countryside, a very nice balance and pretty much the same all the way aound all of London.
The smaller towns and villages around Dunstable are also very nice indeed, very easy to get to although perhaps the need for a car is slightly greater than perhaps St Albans, not necessarily a bad thing - I do know small places like Tring are small and cosy for weekday hangouts to be local and London is just 30-45 mins or so away (depends on how fast you drive!)
As you say mid 20's then I'd suggest St Albans, but also check out just outside of Hatfield, Potters bar etc
Cheers, DtM! West London & Slough UK!