If you're going as far as Wales, you can head for Laugharne to see Dylan Thomas's home and the shed where he wrote. He's also buried in the cemetery there. And for some authentic Dylan-ness, you can have a whisky at Brown's, the pub he used to go to -- which hasn't changed much since his times. There's also a wonderful rambling old second-hand bookshop across the road from Brown's.
Cornwall is also pretty far from London, but is chock full of literary sites. It's is full of Daphne DuMaurier connections -- her home is in Fowey at Ferryside, and Jamaica Inn is down there as well.
Penzance has a house the Brontes used to live in (and my bf's family once lived there, too!).
Cornwall is also John Betjeman country, which could be particularly interesting this year, the 100th annivesary of his birth. His family lived at Trebetherick and he's buried near Rock.
I'd like to second Andrea's vote for Hay-on-Wye. It's a fantastic place.
In London, I've always liked the Charles Dickens Museum. It was his home at one point and is pretty cool.