Hi OP
I am a Westminster parent of a fifth form (year 9) boy. He is a day pupil.
So far the only residential trip during term time has been a trip to Cumbria with boys from his house and year. The other term time trips have been day trips - we’ll often get an email to say that they are going to a gallery etc that morning.
The fifth form boys have to do at least two LSAs (lower school activities). Quite often these are after school on Monday, Wednesday or Friday but there are also societies during lunchtimes. I think my son has one morning activity before school a week.
Tuesday and Thursday afternoons are for Station (games). No early finishes for my son.
Saturday is a proper school day (although it finishes at 1:10pm unless there is a fixture). My son often seems to have a test on a Saturday - it definitely isn’t a “light” day.
Lots of events run through the year. I’ve been on a history tour of the school, a quiz, there’s been a fair (usually in September but it got moved because of some demonstrations scheduled). I’ve been on guided tours of art galleries given by the old head of Art History.
The parent community is more hands off than at my last prep school but I think that is more because it is a senior school rather than because it is Westminster. The boys are expected to manage their own stuff.
Hidden / unexpected costs - there are school trips during the holidays. If your son wants to go on them, they get added to the bill for the term.