You appear to have a pretty good deal there - I hope you never need the cover again though of course.
In general though that doesn't seem an entirely unreasonable sum for the level of cover offered. I'm not sure that it is particularly attractive for myself currently but it is certainly something to look into, especially if you could get a reduction in your NI of a similar sort of amount.
Then you've got a problem but most likely not much of one since there will be spreading of the risk - if it is anything like the banking system then insurers must have a certain amount of capital behind them (enough to cover X% of the risk at any one time) and the chances of that many people claiming at once is small. There is also likely to be some spreading of the risk through insurers 'laying off' some of the policies to other (usually) larger insurers - it may also end up being government backed as a final failsafe.