Cos uses Scandi sizing which goes:
36 = UK 10/ US 6
38 = UK 12/ US 8
40 = UK 14/ US 10
etc etc.
I've found this to be true across the various Scandinavian (or rather, Nordic as Finnish labels also use this scheme) labels I buy too much from.
This is not the same as French or Italian sizing, and is very confusing which is why I always check the label size on secondhand Cos purchases! Likewise, their XS = UK 6/8, S = UK 10/12 and so on.
Good luck, OP, it's a lovely dress.