This is turning into a supply nightmare.
Supplies of seats that fit available toilets are limited both ways - the seats that are listed on websites are not always available (in stock? = Liars) and there are only limited sizes of pans available from stock, which limits the choice of seat. If they're not in stock, the suppliers won't commit to actually being able to get the items, at all.
And, the company that are doing the refit (who have the patience of saints) will only deal with certain brands.
I'm currently looking at a 'merican brand, made in Korea, supplied by a German company with a UK website & phone number (who say they pay VAT in Germany, so can't offer the disability VAT discount. Hmmmm)
Every company I've spoken to (although they won't put in an email) has said the problems come down to a certain project that begins with B, contains an x, and ends in t.