Well I was hoping you'd get a few answers, but no. I am no expert, believe me buyt I am your size. Not sure where you are but here is a list of places I have been shopping at:
Cos Swedish I believe. I love the shapes. They are that bit different. I spent over £400 in store once!! To be fair I bought quite a bit!
Evans - The shop on Oxford Street is better than most. I normally buy shoes and bras. I occasionally buy tops or jeans but hate that most material is viscose.
East - really lovely clothing (I think so anyway) not sure how big they go up to - possibly 18? Maybe 20?
Monsoon - bought loads from them - lovely dresses, my favourite all time pair of jeans, normally great clothing particularly if you like pattens/jewel colurs. I bought a shed load for my summer holiday last year!
Debenhams -(Department Store) has many different shops within the shop (John Rocha , 9-5 by Sienna Millar (or was that at House of Fraser??)and others. I also like their own make stuff The Collection (workwear) and Mantary which is kind of jeans and casual stuff. Good undies.
House of Fraser - cannot remember what they have there to be honest
The much maligned (on here) Marks and Spencers although they are good for some things. I do shop there. I have bought some nice tops and tshirts, a few dresses and jeggings and wide fitting shoes. I normally buy my underwear there too!
Simply Be -is by mail order but also there are a few shops. I nosey in the one in Oxford Street. Haven't bought anything yet but have my eye on a black demin jacket.
Next - though annoyingly you may have to order bigger sizes online. Some shops end at a size 16. Good for work wear. Some top and trousers are nice but you get the occasional 'What is that?' I bought a few white summery blouses last year and a few suits (I was looking for workwear). They have done some lovely dresses in the past.
Laura Ashley -have some fab coats. I have bought some lovely tops too. Good for knits
Phase 8 - but I think only up to 18 (need to clarify this). Available in some department stores as is East and Monsoon as well as their own shops.
Jacques Vert - used to be put off because it seemed very 'mother of the bride' but they have had some good stuff in - I think Debenhams stocks it. One of the department stores anyway.
I think Coast goes up to 18 or 20 but I only look I haven't bought anything from there. There is Oasis too but I am not sure if they go up to 18 or 20. I remember looking at the clothes but cannot remember sizing.
I hope that gives you some idea!