Birmingham isn't easy for me to get there and back from in a day, YoLo, but will continue to be here (a lot!).
I know on S&B there is constant talk about vanity sizing, but I haven't found M&S sizing generous for years (The Pink Dress of Doom excepted.) Their sizing is also erratic. I have three pairs of linen trousers from them, size 16,18,20 and they ALL fit.
Mind you, I am always surprised at the sizes people are wearing. At 15.7 I was a solid 22, whereas many people seem to be able to wear much smaller. Even now at (have I mentioned this ;)) 13.12 I'm mainly an 18, and even that requires a bit of stretch. I did buy a jacket from Sainsbury's recently in a 14, but that was very unusual, and it's edge to edge so doesn't fasten. I also have a couple of smaller Boden summer dresses, but they have a lot of looseness built in to the style. And I'm 5.7, so not dainty by any means.
Basically, cut the labels out of everything! At the moment though, I'm having a mini meltdown because nothing I try seems to sit properly. I think I need to change my style as well as my size. I actually got all upset last night in Sainsbury's changing room because I felt I'd lost 2 stone and looked worse in clothes.