I'm afraid its one of those "how long is a piece of string" type questions, as the conditions vary so much.
I have heard people talking of 20m visibility at St Abbs, however, having dived there for four years now, at various different times of the year, i have never seen this. I have seen 12 - 15m visibility quite allot though, however, as a "rule of thumb" average (and others may disagree here,) i would say 6m visibility is a reasnable figure. The conditions at St Abbs are very dependant on wind strength and direction, if you have westerlies, even if they are quite strong it is not too much of a problem, however if you have any kind of easterly wind you probably won't be able to get wet, because of the swell.
From my experience on the west coast, you can pretty much get in somewhere in any kind of weather conditions. I have been driving up to Oban for a dive with Puffin Dive Centre, and it has been gale force winds and driving rain, and we have still had great dives, as there is always a sheltered dive site somewhere or other. If you fancy wrecks Oban is superb, and i would argue nearly as good as Scapa, with much less traveling and cost incurred. The only probelm with Oban (from my experience,) is that if there are only two or three of you looking to boat dive on the wrecks, the only operator who will take out such a small number are Puffin. Like i say, i personally think Puffin are a good operation, but they are too expensive for me to dive with regularly, but then i guess that comes down to how much money you have to burn.
There is Loch Fynne and Loch Long, but as i said, personally i would prefer to go to the east coast, but again this is to a degree a convinience thing as St Abbs is only an hour away from me.