10 year mortgages do seem to be the longest, although there may be some niche company out there that does longer but I wouldn't be surprised if the rate was really high. Halifax and Nationwide definitely do 10 year fixes.
I suppose fixing a rate for 30 years means you never have to worry about fluctuations but then again, if you look at 30 year rates in the US at, say, 6.5% with no points compared to a 4 to 5 percent 5 year fix in the UK you could be saving at least 1.5% over those 5 years. Quite a lot of money to be saved. Yes, you'll need to remortgage after 5 years but the few hundred pounds spent would be peanuts. I suppose the rate could go up quite a lot but then again it could go down. Unless there's serious inflation you'd be doing well, and if there was deflation you'd really be cashing in on an even lower rate.