According to UK to EU and US Conversion Charts:-
UK Size 8 = EU Size 36 = US Size 4
UK Size 10 = EU Size 38 = US Size 6
UK Size 12 = EU Size 40 = US Size 8
UK Size 14 = EU Size 42 = US Size 10
UK Size 16 = EU Size 44 = US Size 12
UK Size 18 = EU Size 46 = US Size 14
UK Size 20 = EU Size 48 = US Size 16