how I hate that phrase! In a branch or in branches but not in branch! We don't say in shop or in hotel so why in branch when referring to banks or building societies?
Just seen your post and you have said exactly what I’ve thought for several years. I first noticed it when Julie Walters did an advert for a bank (Lloyds?) and I thought how odd it sounded. Seems we are alone in our thoughts!
we used to do things in house. (Not meaning a house literally but within the organisation.) I think it started out as a compound adjective, in-house preparations and so on, but evolved into a free-floating adverb. Quite an old one too.
^^^ The OED recognises that usage, Ginge, but describes it as 'pleonastic', thus essentially agreeing with you:
https://ibb.co/sybr3bX
Missing words can be just as annoying too. I hate "Up to Half Price Sale". The wording implies that nothing in the sale is offered at more than half of its regular price. (It should be "Up to Half Price Off Sale").
The relatively recent usage of premise as the singular of premises is a perfect example of one of the ways language evolves over time, in this case, due to the fact that it just sounds like that's what it should mean". Licensed premises, so why can't we have a licensed premise and, by extension, why can't any building be a premise? :-)