These are species with at least one recorded territory in United Kingdom, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in United Kingdom, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Columba palumbus

Corvus corone

Erithacus rubecula

Pica pica

Coloeus monedula

Chroicocephalus ridibundus

Turdus merula

Troglodytes troglodytes

Larus argentatus

Carduelis carduelis

Cyanistes caeruleus

Prunella modularis
The reference database has no regions filed under United Kingdom.
The field guide covers United Kingdom on the map, but none of its species have an entry written for them yet.