Have you considered Fountain Pen Network? The forum is offline temporarily at the moment, but there were plenty of UK sellers on there when I was avidly collecting fountain pens. Beware eBay, unless you know what you are doing. I bought a P51 frankenpen from there. I still like the pen, but a purist might not.
There used to be fountain pen fayres up and down the country. Obviously a problem until CV19 goes away though.