I think you are going to struggle.
Apple was encouraging retailers to shift the stock a while ago, so most primary resellers have got rid.
Unless the source of the US stock is made available to the official UK market, you’ll be stuck with secondary sources (see above).
Refurb might be the best bet – the phones will be boxed (but not an Apple box) and tbh, unless you use a magnifier you’ll struggle to spot that it’s a refurb.
Go to the network stores to see what they have. Here’s a Tesco SIM free rose gold 6s for £245.
Tesco Mobile Ebay Outlet
As it’s warranted, the only issue will be the non-Apple box (although the box used is tidy and premium quality, just not the classic logo).