Tempest, quite a bit of Ann Summers underwear goes up to a size 20. Some go up to a size 28.
Ann Summers - Deadly Desire Susp/Brief - Product
Ann Summers - Roulette Brief - Product
Ann Summers - Wonderland Brief - Product
I have all three of those and they are fantastic! There is a huge collection on the website.
I don't know how you are bra-wise, but a few of their lines go up to FF:
Ann Summers - Deadly Desire Bra - Product
Ann Summers - Madame Fifi Plunge Bra - Product
Hope that helps. And I really really hope these links work