I don’t think the UK was ever in Schengen (at least as far as my visa was concerned). That is one bright spot here. Traveling to the UK is much easier than travel to Europe for much of the world. The UK allows for paying for QoS so you can pay a large fee and get a visa soon or a much smaller fee and get one in two weeks. By comparison, Schengen visas from all participants are granted equitably by fee and the wait time is months.

Yes you’re right, of course, I was conflating the ability to move freely within the EU member states with the Schengen Agreement which was not correct. Border control was always necessary between Britain and the Schengen area, it was just streamlined.