For women's clothing, UK sizes are 4 higher than US. A UK 8 = US 4, UK 10 = US 6, UK 12 = US 8, UK 14 = US 10, UK 16 = US 12.
Men's clothing is mostly the same. A US men's M is a UK M, and chest sizes (38, 40, 42) are identical.
Shoes are different: UK shoe sizes are about 0.5–1 size smaller than US for both men and women. UK 8 men's ≈ US 9, UK 6 women's ≈ US 8.
British high street brands (Topshop, ASOS, Marks & Spencer) all use UK sizing by default. If you're a US shopper, always check before ordering.
When in doubt, measure in inches/cm and use that as your reference — body measurements don't lie.