Quick note on current travel with Eurostar, the business Premier lounges are back in action since late July and are offering a slightly more limited spread. Nuts/biscuits/crisps are covered in plastic and less varied than before; the usual soft drinks and beer/wine are available (didn't see any spirits but I wasn't looking for them); the cocktail bars in both London and Paris are shut, sadly but understandable.

I travelled to Paris when London was still in tier 3 so I don't know if there are questions asked.

On the way to London UK Border Force check for all the Passenger Locator forms at UK passport checks (or before the e-gates) in Paris and hand out a small paper receipt to hand in upon arrival in London. It seems there were a few people sent down another lane for for secondary questions but I got lucky.

Business Premier and Standard Premier catering are pretty much back to normal, Piper Heidsieck flows in BP.