Will most universities let you defer?

[Newtothisunichoosinggame]

My DC (yr 12) wants to study History of Art the ones he's currently very interested in are UCL, the Courtauld, Glasgow and Edinburgh (he wants to be city based and loves London and a Edinburgh). He also wishes to take a gap year and go to stay with friends abroad (he's just experienced a very significant bereavement and wants time out to get his head around it) but he also wants to apply when still at school so that he will have their help and support. I'm about to book the flight to Glasgow for their open day but then wondered if most universities let you defer?

Yep - he just needs to tick the right year of entry box when he applies.

Yes, as far as I know pretty much all of the Unis are happy to accept applications for deferred entry for the vast majority of courses. I believe that Maths is a notable exception because admissions tutors don't want students Maths skills to go rusty.
I don't have specific experience of History of Arts courses but I would expect that they would view your son's application for deferral more sympathetically than a standard deferral due to his personal circumstances. It might be worth a quick phone call to the department concerned before you book tickets.