Shipping to the UK had been broken for a few days. After some debugging, I figured out that our shipping quote provider had stopped accepting decimals in the package dimensions as of 20 December — but only for destinations within the UK. Shipping a 42.0 cm long box abroad was fine, but it had to be 42 cm for the UK.
Also fixed shipping to Northern Ireland, which had been broken for two weeks (an upgrade had overwritten my old fix).
Everything works properly again.