It will depend on how much you are selling stuff for. But a mixture of general lose change 5, 10, 20, 50 £1 up to about £30 should do you not too much of the small stuff. But most people who go to them also tend to take small change with them as well.
Good luck let us know how it goes, last one I did was about 14 years ago just as we were moving house, made about £70 for about 3 weeks in a row.