Same way as everything’s being enforced to date, by public shaming and snitching busybodies.
Reports this morning say two workers in the Bolton area were issued with £1,000 FPNs after returning, respectively, from Spain and the Netherlands, going back to work on Monday, and being dobbed in by their colleagues.
And presumably there will be a load of Karens outside the pub at 10:01pm tonight with their phones switched on ready to be "shocked" and "disgusted" on Faceache/****ter/Mumsnet about someone locking up a minute too late.