Well, to be percise, the zombie attack survival scenario is more flawed than most of the others.
Nuclear meltdown. If mankind disappeared or was alrgely taken out by the zombie menace, then no one would be around to look after the many nuclear reactors scattered around the UK (or the world). Now I read an article last year (in the New Scientist magazine - it was a great article on what would happen to Mother Earth if mankind just vanished overnight - in short we'd be forgotten very quickly and any trace of our civilisation would be pretty much 100% vanished within a very short amount of time). This article put nuclear meltdown of the reactors taking place about three days into mankinds disappearance.
As such in a 28 Days Later type affair we'd actually have about 3 days to find somewhere safe before mainland Britain experienced 17 Chernobyls!
Maps of Global, Regional & National Nuclear Power Reactors (International Nuclear Safety Center Website)
Now the problem with this is that the only really safe places will be the tunnels and fall out shelters. For those of us living in London many udnerground stations would usually provide some sort of cover from the initial fallout but these will more than likely be zombie-filled. Obviously barricading yourself in a high rise would usually be a good initial option against the zombie horde but exposes you to the fallout during its most dangerous time.
It's possible to try and get to one of the nuclear shelters scattered around London but most people won't have a clue where these are. There are, of course, hundreds of underground structures that one could utilise if you knew where they are located (Check out Subterranea Britannica for some pointers). Of course this does mean venturing out into the chaos, but it's either that or start glowing!
Obviously without the zombie menace you are, comparitively, better off as you will have less problems getting to underground shelters and picking up supplies, etc, en route. Remember you will have three or four days to gather as much as possible before things start going boom! This obviously will also affect all water supplies so you'll be living off bottled water from then on.
As a nice aside, the Chernobyl disaster actually only affected 5% of the nuclear core. 95% of its core remained stable. We'd be looking at potentially 17 total meltdowns as no one would be around to help stem the problem and shut the system down.
Makes you wonder if trying to get to the shelter is actually worth it...