Fasthosts have (for some time) adopted a policy whereby domains are automatically renewed for your convenience (as they put it), but as you say, in practice it means if you don't opt out it will get renewed.
I've just had another bad experience with Fasthosts - this time, they've renewed a domain and taken payment for it, without notification that this was due to happen, and are refusing to refund it. It's only a few pounds, but that's not really the point. If you fleece every customer for a couple of quid, you'll soon make a fortune! (particularly if you supplement your income by selling customer email addresses as ChrisP suggests!