Some areas of rural Britain would be worth avoiding, not least because a potent threat to our survivor would come from Britain's two dozen or so remaining nuclear reactors, mostly dotted around the northern and eastern coasts.The best last-human-on-Earth novel I've read is Earth Abides, by by George R. Stewart (cover shown above). Link (Via Spluch)
If their staff vanished, and as back-up power supplies failed, a real danger would be that one or more would go into meltdown as cooling pumps failed.
The equivalent of several Chernobyls could render big areas of the country uninhabitable. With prevailing winds as they are, it would perhaps be best to head for the westernmost parts of the country or even try to escape Britain altogether.
Assuming our survivor avoided this radiation fall- out, hygiene would be an issue - if he chose to remain in the city, he would find the lack of mains sewage and drainage a problem after only a few days, as the pumping stations failed, while rivers would probably be too polluted to bathe in.
The only hot water would come from a stove, and washing oneself and one's clothes would be a chore (fortunately the world's shops are full of many, many lifetimes' worth of clean clothing).
As for our survivor's health, the lack of any other people to spread infectious diseases would be a blessing, but the risk of accidents would be a constant worry - even a broken limb could quickly prove fatal if the injury was not dealt with correctly.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. Come and hear Mark speak at the ALA conference in Chicago on July 1.