Welcome to the death of culture!! No really I live there .
My best advice is if you have a car, leave the centre and go to the villiages. Woburn to the south and Olney to the North have some excellent pub/resturants which could be worth a look- there are two excellent pubs and a good resturant in Woburn where I suppose you'd be looking at about £40 a head with decent wine.
If you had to eat in the centre, I am afraid that it is almost all franchises although the Taipan Thai resturant in the theatre district is pretty good and would be about £35-40 a head for a three course with wine.