you could go for senegals, palmas palmas, palmas polli, albino senegals and ropefish, there are other smaller species as well but they may be difficult too get hold of in the uk. Ornates, lapradei and endlicheri are also available but can get quite large and most would reccomend keeping them in a larger tank but i reckon they'd be fine in your tank due too the footprint.
They spend a fair but of time on the bottom but do come up too surface for air and sometimes just cruise about, you'll find the more you have the more active they become.