The point is that there was essentially NO racism directed against Black people living in Britain until after WW2. There had been a resident Black population, mainly in southern England, since around 1500, but there had never been anything remotely resembling segregation. Britain did run a large part of the slave trade, but it left Black residents of the UK unaffected. The US white GIs were the first examples most British people would ever have seen of anyone insisting that Black people should not have the same rights as themselves.

Remember that owning slaves was made illegal in Britain long before the slave trade itself was abolished (officially in the 1770s, de facto decades earlier) and there was never any Black slave economy in Britain - a few domestic servants, that was all. (There was, however a white slave economy - coal miners were serfs until 1799 in Scotland, and Thatcher's treatment of the miners was simply the slaveholder mentality in a different sphere).

The situation would inevitably have changed in the post-Empire-Windrush era, but certainly wouldn't have got as bad without the infection of American racist ideology.