Onan
thermals do not stop at cloudbase - cloudbase is where the atmospheric water vapour starts condensing, dumping latent heat of evaporation and hence the upcurrent inside a cumulus cloud is much stronger than the thermal below it which started the cloud forming in the first place.
On the basis of what goes up must come down, the air around the outside edges of a cumulus is likely to be descending
The lift stops inside the cloud at the top, where temperatures have equalised.
Pre and early post-war european glider pilots used to do heroic climbs in cloud (Derek Piggot did 16000 feet in an open cockpit T21 and Peter Scott taught himself instrument flying while achieving Gold Height (3000 metres height gain) in a cloud). The old wooden gliders were very forgiving and had fully speed limiting airbrakes if it all went to worms.
Cloud climbs in the UK are not so common these days because modern glass gliders can go past VNE even with the airbrakes out and their performance means that huge distances can be done without resorting to cloud climbs.
I have no detailed idea of the legalities (or otherwise!) in the UK but the fact is gliders do frequently operate at cloudbase (the sport would be impossible in the UK if they didn't!) in Class G, especially along those lovely things known as cloud streets, so if you are bimbling along in the spamcan, keeping to the VFR rules about vertical separation from cloud is a very good way of avoiding alarming meetings with the non-noisy type of aircraft.