Not wishing to be a smart-arse or anything, but...
Why are you all so keen to severely limit your audience and cut down on your sales?
Why would it do that?
There would be nothing to stop people listing higher value books in the main sales forum (in fact in practice I am sure they would be more likely to do so) and post links in the UK thread.
I think we are mainly talking about lower value books and sets here. At the moment those are not being listed at all by UK sellers, so there's no audience to be limited and no sales to be cut. I'd argue that a UK sellers' forum would do the opposite of what you say.
You list books in the appropriate Marketplace section, every UK buyer can see them and so can everybody else.
You have a UK-only section and every UK buyer can see them...but very few other people will venture there.
Lower value books sell internationally...I know because I've sold thousands...and the only problems you have is that (a) nobody else in the UK bothers listing them and (b) few UK sellers, barring Garry and Andy, have a long resume with international buyers.
Before we reinvent the wheel, why not try to sell under the current circumstances and see how it goes? A round-robin PM would be useful for everybody to link their latest thread in the Marketplace - but then I'd suggest you also include all of our European colleagues, as shipping from the UK to mainland Europe is a hell of a lot cheaper than from US to mainland Europe.
One other potential down-side is that an anti-Brit sentiment can be generated by us requesting our own section. If you've been here long enough, you'll know that anti-Brit sentiment bubbles just below the surface with many and I could possibly see a *spoon*-storm over this that will not help us. Just sayin'...