You might like to read this about the Bicviewtopic.php?f=6&t=21530
Actually, the Bic magnum (not super magnum) is probably fine - I noticed some pretty good deals with wetsuits and leash and bag etc thrown in for not much money really.
Also have a look at NSP boards, these have worldwide reputations for being tough and good beginner's boards and arent too expensive, I think (dont know UK prices); although probably more than the Bic.
But before you splash out on a new board, check out a few surf shops and not just on line/ebay. Most shops also have second hand boards - call up the ones near where you live (if any) or near where you surf and ask them what they have. The benefit is that you can ask them to repair any dings (cracks/holes) before they sell it to you (as part of the purchase price), so you don't have to do your own inspections.
as Jaffa mentioned, anything around 9ft will be fine. Aim for 22 inches wide (or more) and I would say perhaps about 2 3/4 inches thick, give or take. Most longboards fall within these dimensions anyway.
Three fins is called a 'thruster' set up - almost all longboards nowdays have 3 fins. Some longboards will have only 1 fin (which is usually quite long) and some people think 1 fin is more stable than three anyway.
This is a good summary of boardshttp://www.newquaysurfer.org/2011/03/05 ... surfboard/
Good luck - it can take a while to pick or find a board and you wont know whether its the right board until you have been surfing for several years...but you will likely have this board for 3 or 4 years at least and, even if you move to a different board, may well have it around for small wave days or for visitors etc. So its never going to be a waste; like anything the cheapest is cheapest for a reason; but that doesnt mean it wont be perfectly suitable for your purposes.