clarification from accountant:

HMRC classify you as a trader *if* you are doing 'hundreds of trades daily/weekly'

if you aren't, then you are liable only for CGT (threshold £11,300, rate = 10% CGT for lower rate taxpayers, 20% CGT for higher rate taxpayers) for each disposal.

Don't take my accountant's word for it though, although this info should be helpful to us UK people - Always check with your own accountant.