Zopa is a great idea but you are playing on a field tilted against you. Put your money in a bank, and the government guarantees you'll get it all back, even if the UK economy tanks and many people that the bank has lent your money to default on their loans. Also the bank is too big to fail, so it makes reckless loans and drives loan rates down, knowing it cannot go bust. Bankers! Lend your money through Zopa, and you are on your own. Yes, you get more interest, but personally I don't believe you get enough to compensate for the risk to your capital, especially after it's taxed.
Silly question of the day: if the government takes 40% of your interest in tax, shouldn't it reimburse 40% of your losses if things later turn sour? :-(