Unfortunately, the UK distributor Cleghorn Waring ran out of Brydon spares a few years ago. If you were very lucky, you may find a chandler with some in stock but I think it unlikely.
It's probably still worth sending them an email just to confirm.
Fox's in Ipswich have spares kits. They are Dutch and called something like "generic toilet service kit". I have one but its on the boat so can't give you exact details. About 25-30 GBP as I recall and have every seal and valve. I tend to replace them all every year if they need it or not since I know relations with the admiral would turn sour fast if the heads didn't work.
Confirm Brydon Boy , no longer in production and "official" spares no longer available, but have also benefited from Fox's Dutch spares kit-I think the letters RM are somewhere in the label. Managed to keep our venerable version going-much better quality than the present offerings.
The spares kit for the RM69 toilet also fits Brydon Boy. RM69 spares kits are widely available from most online chandlers if you can't find one at your local place. RM stands for "Raske & Van Der Meyde", the Dutch manufacturers.