Much of my important data exists on several different computers and in hard copy as well, though not always as a result of deliberate backups. Pretty much all my important data could be rewritten, by me, although it would be time-consuming and probably not produce an exact replica of what I wrote the first time. (Though this wouldn't necessarily be bad.) The irreplacable data I've lost in the past has been mainly things like lj or Usenet posts, or Word documents, which I was typing and then lost in some way e.g. the PC crashing unexpectedly. I just had to type them again, probably not quite the same way as they were in the first place.

Some of my data e.g. my thesis, has been emailed to people in other countries, or would exist in Google caches visible from other countries, so it should be recoverable even if the UK were destroyed! But I think actually I might be more worried about other parts of my life than about recovering my thesis if the UK were destroyed.