Its possible to do a lumpsum into a private pension, many of the big life assurance / pension providers offer these.
Unlike workplace pensions, where employer will advise HMRC of the employee contribution, you need to claim the value of amount you contribute into the private pension on your self assessment. Forgotten the box number it goes into, but there is a section on tax reliefs and one box is for (non employment) pension contributions, which from memory is first box. I think you have to add 20% because the pension provider will claim basic rate tax relief (but I think the note explains what to do on self assessment form)
Remember if you have to wash/launder any work clothes with company name or logo on it there is a box in same tax relief section for that too. Why pay more tax than have to.