Fallout: Bognor Regis
...I'll go get a notebook.
I've just done the maths. Fallout 2 covers a square about 300 miles high by 300 wide. The equivalent square in Britain would cover everything south of Newcastle and would still be smaller due to all the water. In contrast, Fallout 3 is roughly a square 14 miles high and wide. If we take the latter scale for Fallout: Bognor Regis, you'd have the eponymous city in the south, with the South Downs as the northern border, Chichester to the west and Littlehampton to the east. As the area is mostly fields, you could faithfully render every village as an explorable landmark.
Of course, if we were really making an English Fallout, we'd pick an area more densely packed with actual landmarks. London would be obvious but too much hard work. I suggest the are around Newcastle! You'd get in distinctive landmarks like the Angel of the North and South Shields Pier, with plenty of mostly empty ground in the middle for encounter-filled travelling between the towns.
The great thing is, the place names already sound like they're from a post-apocalyptic wasteland. Nuke-Castle (or is that too on-the-nose?), Low Fell, Sunder, Lamesly, Jarrow... and uh, Kibblesworth. You could even use the River Tyne as a natural northern border (with only central Newcastle, south of the river, accessible)