I am no longer as folically blessed as I once was so I have no need for a barber these days. In the past I would tip via the "keep the change method". Once they accepted card there was no change and if they wanted to charge £10 instead of £9 they could.
The tipping culture in America is horrendous. It's a way for businesses to avoid paying proper wages and workers expecting tips at a certain level makes the whole experience awful. Here in the UK we have minimum wage laws so there is no need to tip. I also find it bizarre that there are certain jobs that receive tips where others don't. Waiting staff in a restaurant for example are taking home more than the people cooking the food or cleaning etc with tips but I don't see why. They are all part of the team. Food delivery apps are now charging higher prices than going direct, then there is a delivery fee then an admin fee and now they are asking for tips as well. Tipping should be reserved for exceptional service above the expected level.