La senza Boux Ave they have both been good with me with bra fittings i dont like m&s either :-/ debenhams are also very helpful they seem to be in the know if you tell them what you want from the bra they will point you in the right direction as to what will be comfy and suited to you :-)
M& s should. Be best place to start. They train their staff on measuring pll up for bras. And they have a goodly range too
Just remember, you can be a different size in different range and styles.... It is a bit subjective like dress sizes can vary from one store to another.
If you feel you might. Be on the smaller side,you can get padded ones that can give you a little more.
BTW you don't have to strip for a fitting. You can slip each bra on, and get then go check the fit once on
Boux Avenue have some great videos on their website to enable you to fit your bra yourself. And you know what fits better than any staff member with a measuring tape can tell you.
https://www.bouxavenue.com/fitting-service
I have had good fittings, and bad fittings. I have very irregular shaped breasts, with one being almost 3 sizes smaller than the other. I have left shop assistants exasperated, but find that doing it myself at home always helps and I can look at different options to suit.
Best of luck.
If anyone laughs at you when you go for a fitting, complain about them because that is disgusting. Not everyone feels comfortable enough to get fitted. It is that person's job to make sure that you're wearing the correct size, not to criticise you for making a mistake.
Sorry about the little rant, but I really hate poor customer service and mean people.
I personally like getting fitted in Victoria Secret's because all of the ladies in there are lovely, and they write your size on a little card, which is handy if you have a rubbish memory like me, They also bring you a few different types of bras because you will find that your size may change with the different types of bras.
Good luck with wherever you choose to go and remember that no matter what size or shape your breasts are, it is still important to wear the correct fit :)