Hi Caveman, I don't have much information for you about "up north" - I was hoping to do some "pioneering" reports from Yorkshire a few years ago but then my reason for going there moved south. I wonder if you get enough visibility for spearing up there? You don't need a lot but you really need at least 1m - ideally more. The way to find out is to "suck it and see" but perhaps that's not for beginners.
Be careful & cautious, especially when starting out. Look for favourable conditions (flat seas, little or no wind for a day or two before hand) and locations (easy entry and exit, maybe some rocky shore line to follow nearby).
As for stores, I like Apnea.co.uk on Jersey - beware the UK tax authorities recently lowered the tax threshold from £18 to £15 for how much can be imported before duty is due

- also the Royal Mail now charge an £8 handling fee when they collect that duty! Nasty. You can buy from the EU without problem though (try searching eBay with the EU option checked if you want).
There is also Anthony in Paignton, Devon - I always forget the store name...Adventure Sports maybe? There is also a Rob Allen dealer in Dorset Crystal something - pricey though. Probably worth visiting your local dive store - although some SCUBA divers are a bit funny about spearing, many stores carry some spearing related gear. Be warned though, SCUBA gear is different from spearing gear. For example spearos tend to favour: low volume, twin lens masks; simple J-tube snorkels; long slimmish fins with footpockets; rubber belts with Marseille buckles & D/Alton-style weights; simple, close-fitting open-cell wet-suits with hoods but without zips. Many/most folk start out with sub-optimal gear - SCUBA masks & cheap surfing wetsuits are common beginner gear - best not to spend money on that but if you already have it or have other uses for it (e.g. SCUBA diving/surfing/kayaking) then by all means make do for now but understand that it is not as good - sometimes by a long chalk (e.g. the wetsuits).
Omega3 reckons parts of Scotland have lobster running around like rats, might be worth a visit. It used to be thought that Bass didn't get that far north but I think that is no longer true with our "improving climate" - but they do seem to prefer the warmer months. Yorkshore is famous cod (and was once famous for herring & great blue fin tuna- those day are long gone though). Not sure about grey mullet. There are likely other species around - chat to the anglers.