It's a good question.
Basically, if you are just ordering a drink, I don't tip. Indeed, many pubs now have an app. You sit at your table, order a drink using the app, and your drink is delivered. Similarly you can order food in the same way.
In a more traditional pub, it is more nuanced. Sometimes you go to the bar to order, and the food is delivered to your table, other times a server will come and take your order. If the former, probably no tip. If the latter, when there is a bit of a chat etc perhaps a few £, but the tip will be the same, irrespective of the amount of the bill.
To be honest, the same would apply in a gastro pub. Just because the meal is more expensive, does not mean that I should tip extra.
I think that what needs to be recognised is that staff in the UK and Europe, get paid a decent wage, plus paid holidays etc. They are therefore not dependent on tips to survive, which is not the case in the US.
In the US, I understand that staff are grossly underpaid, and require tips to survive. So, I tip accordingly. Elsewhere, staff are paid realistically, and therefore a concomitant tip is appropriate.