m&s have nice basic t-shirts that are plenty big enough, most in the 'normal' ranges go up to 20 or a 22. i love monsson too, but as someone else said the bigger sizes go very quickly, unless you live in chichester where it seems to be all the 18+ left and all the 10/12 gone so always checkout chichester when we go there.
for cheaper more fashionable clothes i use new look (inspire range), they have some lovely going out tops, and their jeans come up huge so always a confidence boost to walk out with a pair of jens 2 sizes smaller than your regular size. umm MK one do a plus size range (up to a 24 i think) but there stuff comes up small, so pees me off when i have to buy a skirt at least one size bigger.
primark do some things up to a 20/22 i got three lovely tops there yesterday, quite plain v neck, long sleeve jersey t-shirts with a lace panel behind the v for £4 and they are huge so as well as big enough they are long enough too. i also brought a skirt for £8 and some boots. you just have to be prepapred to serach through the racks.
also debenhams, the casual club, maine and the collection have up to a 22 and they are generous sizes, got a lovely cardi similaar to one in monsoon for £75 in debenhams for £30 and denim skirts for £20 plus basic tops for less than £10. some if it is a bit granny but if you look carfully they have some very nice stuff. also noticed that the john rocha range has statred to go to a 22 in certain items.
apart from that catalogues, like simply be and freemans, they are good for wide fitting shoes as well.