Old Ticket Office in the "North wing" of Cambridge station is okay if a bit pricey.
Don't go in the Station Tavern nextdoor, it's horrible even if it looks smart from the outside. Even their keg beer tasted off!
Earl of Derby is a 5 minute walk away near Hills Road bridge and is alright for a GK place, a good spot to wait for a train while avoiding the crowds around the station.
Peterborough station has The Brewery Tap (Oakham) a short wander (five minutes or so) away across Bourges Boulevard, with a nice cosy micropub called the Bumble Inn five minutes further walk opposite Queensgate bus station. Both pretty tidy places, the Bumble offers a good range for its size. Brewery Tap is usually exclusively Oakham and I'm told does fairly decent thai food too.
The Beerhouse is a short walk from Bury St Edmunds station, one of a few pubs in the area owned by "Brewshed" who do some decent quality beer.
A nice spot when changing trains from Wymondham "mainline" to Wymondham Abbey used to be the Green Dragon but I have no idea what it's like now, believe it got new owners last year.
Hamilton Hall 'spoons at Liverpool Street is fun if you can get a table upstairs to people watch from, otherwise bog standard spoons.
Not a huge distance from Euston or Kings Cross is the Somers Town Coffee House (it's a pub despite the name), tucked away off the main drag and far better because of it. Served a good pint and had a nice atmosphere in there. Far better than the Parcel Yard in KX itself which seemed to be overcrowded and serving bad pints when I went in there one lunchtime.
Euston Tap is good fun. Nice in the summer but entertaining in the winter - back in February myself and a mate had just bagged the one table without an umbrella as it started to rain...
If you have money burning a hole in your pocket, Mother Kelly's taproom near Bethnal Green station (under the rail arches) is worth a visit although its prices are high even for London.
Has anyone ever tried the little Platform 1 bar at Ely station?