As it's a reasonably large amount of money you would probably benefit from talking to an independent financial advisor.
If you want all your money to be very safe and aren't too worried about getting a big return on your investment a combination of Cash ISA and fixed bond sounds perfect. You can only put £5640 in a Cash ISA this tax year but, if you stagger similar amounts in bonds that mature in 1, 2, 3 years etc., you can use those to fund ISAs in forthcoming years.
If you don't need the money straight away and are happy to consider a small amount of risk for part of your money there are other options for long-term investments. Stocks & Shares ISAs in the form of unit trusts, tracker or managed funds, for example. If you have a pension you could top it up and get the tax-relief on your contributions. If you have anything outstanding on a mortgage, paying that off could make sense.