As a woman of large norkage,and ex-nurse who worked on a breast surgery ward where we fitted prosthesis,I am OBSESSED with people having the right bra.Tips:
No need for a measuring tape.The support from a bra should come from the band-not the straps.The straps just hold the cups in place.The band(main body of bra),should be flush to the skin,not ride up,and if you can fit two fingers in under the bra at the front it fits.This may feel uncomfortable at first if you have been wearing bras too big.
The cup should encompass all the breast.The breast should be moulded in the cup.Not sitting at the bottom of the cup,or pushing up out of the cup.
If you are using underwired bras,where the wires "meet" at the front should also be flush to the skin.
Personally,I have never been able to find a bra to fit me at M&S,Debenhams or any other chain store.I live in Ireland,but have sister in London,and go to Bravissimo.But I go to Rigby and Pellar in London once a year,as I find the fit is second to none and the bras last years if you handwash them.If you are big boobed,I don't think you should scrimp on bras,they make a huge difference to your shape,and good ones are so comfortable!I'm a 34H!