Tax returns are not that onerous. If you don't fancy doing it yourself, a local bookkeeper will do it for you for £50 or so as long as you keep good records. However, if you ever find an investment scheme that offers to take your money, pay you lots of tax free income and deal with all the paperwork for you, you should hide your cheque book and run away.
Peer to peer lending is a lot more risky than investing in a high street savings account and any profits/gains will be taxable. Having to submit a tax return, to me, would be the smallest of worries. How would you feel if you sent of a substantial investment and then, six months later, you were told that because of economic circumstances, the return for investors had changed and then, after another six months, you were told that the people who took your money were having financial problems?