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If you go up a back size, you go down a cup size. For instance, I measure a 30c but so hard to find on the high street so I go for 32b instead (if you hold them up side by side you'll notice the cups are exactly the same size, same as 34a cup being the same as a 32b).
28C as a starting point (they are rare but M&S do a few) You might also want to try 30B and 30C because if you can find ones with the right cups it is very easy to adjust the back.
Up a cup down a back only works up to a point - really when you are between sizes. I've seen a few people who go up 2 or 3 backs (and down a corresponding number of cups!) and that's an epic fail normally!