Is there any reason I should brew one before the other as far as the yeast health goes? Will the color of the scottish ale yeast cake affect the pale ale? Should I wash the yeast or just repitch onto the previous batch's yeast cake?
These gravities are pretty close together so it doesn't really matter much which you brew first. As long as the yeast is sitting in the base of the fermentor as a tight cake you can pull more than enough beer out without it effecting the next one too much. And you don't need to pitch the whole cake, just half a pint or so. I have done this with good success and I don't bother washing the yeast although you can if you want.
I once pitched yeast from a spiced holiday ale into an American cream ale. I had no noticeable carry over of spice or color change.
It's mostly a toss-up, but I'd suggest the 80 shilling as the first beer. The APA will leave considerably more hop residue in the fermenter. If you do the APA first, you'll either be adding a lot of spent hops when pitching the yeast, or you'll need to wash the yeast cake.
It is generally recommended to brew low-gravity beers before higher-gravity ones when re-pitching yeast. It is often said that you should also pitch light before dark beers. I asked a question about this recently. Color transfer isn't usually a problem, but you should pitch low- to high-gravity. With this in mind, I would brew the Scottish Export first, then the APA.
Those being so close in gravity, I'd not worry so much about the gravity and do whichever is lower in bitterness first. I would probably not bother with the wash unless you had a lot of hop material in the trub. Otherwise it is probably not worth the extra risk of contaimination.