There's an argument here to be made about reality. There are some vulnerable banks out there, in order:
B&B - toast, get out while you still can, have you seen the share price.....
A&L - vulnerable, same issues as B&B
HBOS - not very vulnerable, but in a fair amount of trouble.
I would argue that if any or all of the above fail, the same protection as NR would kick in. Failure to do so would result in a much wider banking collapse as everyone puts cash under the mattress. The government will do everything in their power to prevent this, up to and including just printing the cash.
There's a group of less vulnerable UK banks which can be summed up as "the rest of them", HSBC being the strongest of the lot. These are the big boys where an NR style rescue might not be possible. On the flip side, if any of these let go, then it's good night, game over, end of the financial world as we know it. So it doesn't matter. If we were in a situation where, say, Barclays or HSBC failed the fact that you had your cash in the Nationwide would be irrelevant as your cash would be a string of numbers, nothing more.
Summary:
Stay out of Iceland and the vulnerable banks. Everything else is OK. Inflation is your bigger enemy as it is currently dissipating your savings at about 7% per annum assuming you are getting 6% interest and paying 40% tax on it.