Beer recommendations — hmm. If your friends are coming to London, then I’d recommend anything by the Fullers brewery — the one I suggested to your dad is called London Pride (it is a bitter beer, so quite dark and strong-tasting) but they do a wheat beer called Discovery, one called Honeydew as well — both of those are very nice. You’ll usually find them in bottles rather than draught/tap.
Here in the UK there is a weird system where the majority of pubs are tied to a particular brewery so they can only sell that brewery’s beer. So there are some pub chains where the beer isn’t that great — Wetherspoons pubs are generally to be avoided, they are nasty, and Sam Smiths’ pubs are often cheap but the beer tastes cheap as well.
Or there is always cider. Last summer it was very trendy to drink cider out of bottles (magners or Bulmer) with ice — a bit weird, no?
I would recommend to your friends to check out a website called Beer in the Evening. It has good pub reviews, maps, etc. I’d trust it, for sure ..