Hi Carlo, as mentioned of the other thread, you could check the British Spearfishing Association for Clubs:
British Spearfishing Association - index.
Several forum members are members/officers of the BSA.
Or the London International Club:
--- S P E A R F I S H I N G ---> Home of the London International Club
...both sites have useful links to other sites too.
Also check out:
Spearguns by Spearo uk ltd finest supplier of speargun, monofins, speargun and freediving equipment -- no clubs, just a good place to find gear & some good, concise advice for beginners.
You could also try Nottingham University & other colleges in the city, students often organise clubs around all sorts of interests (they might affiliate themselves with the BSA). You could try starting your own. Some pools offer snorkelling sessions (mine does), might be a good place to start.
Looking at the map: http://www.multimap.com/maps/#t=l&map=52.96667,-1.16164|8|4&hloc=GB|Nottingham
Nearest locations look to be N. Wales (talk to Pastor), E. Anglia (reputed to have decent rod fishing), Blackpool area & Lincolnshire (which I know almost nothing about). I have only come across one spearo on the forum from the North so far. Looks like you will need to travel some way to take part. Plenty of new territory to explore. I did hope to explore the Yorkshire coast - but that looks unlikely to happen now.
Spearing activity seems to be mainly on the south coast & Wales - at least judging by contributions to the DeeperBlue.net forums. The season will be longer further south, with gulf stream warming. There is probably good spearing to be had elsewhere though (Scotland?).