Scapa Flow is great, and you really don't need adv nitrox and deco to get the most out of it. Scapa has this deep, dark and dangerous rep, and it is not so. The main wrecks - the four cruisers and three battleships from the German WWI fleet - lie in 12-46m, so using a 15-litre single with a three-litre pony as bailout still gives you plenty of time on the wrecks and you don't even have to go into deco. Using 32% or 36% nitrox gives you longer on the wrecks, but there are no real currents to speak of on the wrecks (only on the blockships in the channels, or sounds), the vis is usually anywhere from five or six metres up, and as you can see, they aren't that deep.
Scapa Scuba is the PADI centre up there and they offer guided dives, so if you are a little daunted about jumping in and dropping on to a 600ft, 36,000 tonne battleship, you can have a guide who will show you all the cool stuff and navigate the dive. Makes sense if you are travelling and diving alone.
I have taken two friends up to Scapa for their first British sea dives, after about 50-60 build-up dives abroad and in local quarries, and it is a top spot, but nowhere near as deep and dangerous as its rep.
The Sound of Mull and Oban area off the West Coast as good, as is the east coast around St Abbs/Eyemouth. The latter, in particular, offers some nice, shallow shore dives.
Mark