The UK was never part of Schengen, but it was part of the single market. So while passport/ID controls have always been performed on the UK border crossings with other EU countries, customs controls had not – until Brexit.
I am not aware that customs controls are made in the passport area, at least not between London and Paris. It is done at the exit of the terminal, in a non blocking way like in UK airports (and always was, they were already looking for illegal products before Brexit).
