.Which is to facilitate the (usually) 4 runs a week of 9O57, the early evening arrival from Marne-la-Vallée–Chessy which has French Police aux Frontières checks before departure but which has no UK Border Force checks there, so checks are done on arrival.
This causes the Mitie security personnel at St. Pancras no end of grief as they have to segregate arriving uncleared passengers from those from other cleared trains arriving at similar times, and a nearly-full 373 still has the queue backing up onto the platform. They also have to search the platform after everyone gets off in case there are any clandestines waiting for the next train to arrive when they could mingle with cleared passengers.
(To pre-empt the question as to why Eurostar is willing to take the risk of having to return inadmissible passengers to France on this service, it is incredibly low-risk as materially all passengers are British (or at least British resident) families on short trips to Disneyland Paris.)
(To pre-empt the other question as to why there is no UK Border Force présence at Marne-la-Vallée–Chessy, with usually 4tpw it’s not cost-effective.)
(To pre-empt the other other question as to why this is not cost effective but presence at Moûtiers and Bourg-Saint-Maurice which has only 2tpw is cost effective, the 2tpw from these are on the one day, as opposed to the MWFSu usual service at Marne-la-Vallée–Chessy.)
Quite.