I don't know how universal this is, but our BHS has got loads of sloggi knockers in the clearance section, for £3-4 a box...you get 4 in a box, for some of them, or 2 for the control briefs.
They had loads of 10s and 20s and one pair of 12s, but are generous as I fit both and usually need a 14 for pants despite being a 10 in everything else.
I am afraid I bought 18 pairs but it cost me £27, so bargainous
It would normally have been about £13 a box.
Worth a look!