Iamgiulietta, I found before that the sizing in M&S/supermarkets can be a little less predictable. You will almost certainly be able to get something - I think Asda do a range by the woman who created Ultimo which is in that kind of size, and M&S go up to a GG in a 32 back. But, it might be worth taking a look at a few videos (e.g. bravissimo website) on bra fitting so you know what to look for. You've prob seen things like this upthread but it's a big thread so....
Basically:
Back should be horizontal, firm and only be able to run a couple of fingers inside it. Personally, I buy bras so they fit on the loosest hook, as they give a little when washed so this gives me room to tighten them up.
Wires should be completely encasing your breast tissue - you need to scoop everything in from the sides, and then feel whether the wire is lying on or outside your softer tissue.
The wires should sit flush against your chest in the centre - if they sit proud then the cup is too small.
If there is spare fabric, don't immediately assume it's too big (almost did this myself earlier)- whilst the bulk of the support shouldn't come from the shoulders, the straps are often set to be stupidly long and just tightening them can suddenly pull a lot of spare fabric up and away.
and the obvious - there should be two boobs, not 4 :)
P.s. I'd also try a 30 back, in an E/F roughly