£20.000 single payment into pension plan

Hi bengie
Assuming your earnings will allow such a large investment then there should be no problems.
Your choice will be between going with a pension provider, details below:
http://www.money.co.uk/pensions/all-pensions.htm
Or taking a little more control over your own investments inside a pension:
http://www.money.co.uk/pensions/self-investment-personal-pension-plans.htm
One other option you may wish to consider is a SIPP. I attach a link to a company's website that will explain the benefits of this type of pension:
http://www.hl.co.uk/pensions/sipp/what-is-a-sipp