Are there any sperm banks in the UK where you can bank some sperm for use later in life? How much does it cost?

There are most definitely sperm banks in the UK.   The cost depends on the level of service you require, no I don't mean they'll extract the sperm from you.  Remember that prices may include insemination as will as storage.  Your best bet is to call one of these places yourself and discuss your options.  Here are a couple I found in the UK, I think you'll be able to tell which one I suggest you contact first.

http://www.spermdirect.co.uk/
http://www.mannotincluded.com/