Hey Fairy
Debenhams have a good range of dresses, some of them are lovely that go up to a size 20, and some of the sizings are pretty generous too.
They are about £60-£100 though so that's something to consider.
I have just been to Monsoon, John Lewis etc as I have 3 do's to go to, tonight, next Sat and the follwoing Fri so I have needed to get kitted out. I am a size 18 (just dropped from a 20) so can empathise.
Monsoon were expensive and this season not great IMO
John Lewis only go to an 18 and were expensive
Debenhams had some great stuff but £££
I know BHS go to a 20 but not sure what they have for a more formal occasion. Plus the clothes from there tend to lose shape IMO.
I also would say don't rule out Tesco who occasionally get in some gems for the bigger sizes. I have some smashing (and cheap) stuff from there.
Also check out Next online, I saw some brilliant dresses and outfits around £25-£50. I went for one of these actually and DH thought it looked gorgeous. I would say this might be your best bet?
Anyway. Hope it helps.