Welcome to DB. you will find that this is probably the most comprehensive board available. one of its best features is the "search" function
ok, first thing you have to consider is the environment you will be diving in:
average vis, target species, etc.
given that i dont dive UK waters i cant tell you exactly which gun to use, but as a general rule, bad vis and rock working require smaller guns. However you will often be presented with opportunities to shoot free swimming fish that feed on prey neer rocks. this is important because more than anything it will determine your band and shaft setup.
free swimming fish often require faster shaft speeds than rock-work. however since you intend to hunt around rocks you need a thicker shaft that is less viable to bending and will hold its tip. in my opinion i have found that a 6.5mm shaft is a great all rounder, with my preference being a 7mm.
pencil points are nice and sharp till you hit a few rocks and they start to loose its point. for that reason i prefer a tri-cut tip. also, re-sharpening a tri-cut tip is much easier.
overall, i would have found that a good tri-cut 6.5mm shaft, coupled with a 80cm gun with 16mmx2 bands works best for rocks and free swimming fish that frequent rocks.
check
Spearguns by Spearo uk ltd finest supplier of speargun, monofins, speargun and freediving equipment for more information from the retailer. you will find that cressi-sub is right on the money for a starter gun. i started with them and i can honestly say taht they are perfect for begginers. save your fingers and convert your wishbones to either dynema or steel wire, and polish the notches on the shafts.
use the search function and you will find a lot of usefull info.
good luck