I'm sorry you've not been well but your request is a bit vague so sorry but this is a rather vague reply.
The obvious "dressy dress" places on the High Street are Phase Eight, Coast, Hobbs, Mint Velvet, LK Bennett, Monsoon, Anthropologie where I'm sure you'll find something although it won't be terribly exciting, with the exception of some of Monsoon and Anthropologie. Reiss possibly as well
Pricier than "High Street" there's Me+Em and Max Mara where you will definitely find something.
Alternatively go to websites like Liberty, Harvey Nick's, Brown's, Coggles, Matches, Net-a- Porter, MyTheresa , filter for your size and maximum budget. They will bring up what they have in stock but will also give you the names of brands you can search on the individual brand's websites.
Possibly a wild card but mentioning just because it's been mentioned already today but there's heaps of options in this site.
Gather and See
Or more wild cards, Stine Goya, Sister Jane, Ganni?
As for not having another occasion to wear it, avoid anything described as
"wedding guest dress" There's loads of options from all of these suggestions for dresses which are dressy enough to wear to a wedding but which can be incorporated into every day life afterwards.