You can go to the Small Claims Court, to sue for sums up to and including �5000, but to lodge the case it cost about �130 then to file documents with the court, it's an additional �100.
That would be alot of money to sue for only �20 and also the judge might be so irritated by the case, deeming it as petty, he might favour of the defendant, so not only would you lose over �200 lodging the case, you'd lose your �20 too.
Just put it down to experience and don't lend money again.
Have you asked for the money? I'd send a letter to the person saying that they have 10 days to pay after such time you will be apply to the court which could result in a CCJ, them having to pay costs and destroying their credit rating.
That might spur them on to actually pay the money. Then again it might not.