Well, there's this forum of course!
I also visit the below forums, all of which have sales threads:www.myarmoury.comwww.swordforum.comwww.sword-buyers-guide.comwww.livinghistory.co.uk
I'm not aware of anywhere that specialises specifically in selling British swords, though.www.southernswords.co.uk
also acts as a vendor for you, although they do charge a fee (£5 per item last time I checked). You can send them photos and some info on what you're selling, and they'll post the ad in their 'Customers items for Sale' section.
Hope that helps.