M&S fitting is notoriously crap but their bras are actually pretty decent IMO.
The best option is to measure yourself the BOB way and really have a look at what a good fitting bra should look like and what shapes and styles are likely to fit you. Then you can try a few bras on and adjust from there.
I measured 28FF/30F on BOB (and I have pretty small boobs- I'm almost flat chested without a bra but my boobs are quite wide and high so there's actually quite a lot of breast tissue if that makes sense). I wore a Freya Deco 28FF for years but when two of my old trusty ones broke I couldn't find any at a reasonable price.
I went to M&S and tried a load on. I actually came out with a 32E (which is a 'sister size') but it fits exactly like the 28FF, passes the band test etc. When I measure it against the 28FF it's exactly the same size.
An M&S 30 was too tight on the band test, so their bands are a bit smaller than other brands, so maybe their plus four works? However I think the leaning measure is what really makes BOB fitting work, then you really need to swoop and scoop.
So, if I'm making any sense, you could try a BOB measurement and then sister size it up one or two bands to get your M&S size. Eg if you measure 32F on BOB try M&S in a 34E and a 36DD as your starting sizes and adjust from there, following the BOB fitting tips.