I have considered this myself in the past. I think either choose one of the big chains (I can't see how the staff in Costa for example would care or be affected by you sitting there), or if you go to an independent, go out of your way to speak to the staff/owners and ask them if they're happy with it. If they're super busy every day with people coming in to eat every day then they might not appreciate it but if they're not always full I can't see why they'd have a problem with you buying a couple of coffees and a sandwich each day.
Or you could try a pub - likely to be much bigger, a pub near me was actively encouraging this as we came out of lockdown and I think offering a deal of unlimited tea and coffee plus a sandwich for £10 or something.
And again if you went to a national, how about Wetherspoons - huge, and unlimited tea and coffee refills for about £1.99!