I am only going to be partly helpful here.
You can sub BLEACHED all purpose flour for cake flour without any other changes. (I think, upon occasion, Rose will recommend a slightly different quantity, but, if so, it will be specified in the recipe.) Apparently, the bleaching of the proteins change them enough to work without the finer cake flour milling or adding corn starch, etc. The final texture will be a little different, though (not as fine a crumb), but the cake will rise correctly, have a good crumb, etc.
However, I’m not sure if the bleached Gold Medal flour you are finding from your supplier is the same as the U.S. version—you know how manufactuers make things a bit differnt from country to country. With that in mind, I can’t say for sure if it would work as intended, unless the flour is the “U.S. version.”
We have at least a few UK bakers here, and, hopefully, one of then will see your post. You are the first person I have read who has posted about being able to find bleached AP in the UK, so I’m not sure what experience they will have.
Finally, I’ve made Rose’s cakes with UNBLEACHED AP, and they come out with a “homey” texture with a bit of a coarser crumb—depending on the cake, it’s rather nice. Here’s a link to that cake—you can see the crumb, etc.: http://www.realbakingwithrose.com/index_ee.php/forums/viewthread/3054/ If you do this, you will want to decrease the BP by 1/8 t. (or even a teeny bit more, but not much) per layer to strengthen the structure. I didn’t, and I got a bit of a birdbath effect, but frosting took care of that nicely.
Try a one-layer test!
Good luck—let us know what works for you—and post pics of your results if you’d care to!
p.s. We also have some good scientific folks who will be able to answer your protein questions!