You should be giving a good service anyway, not expecting a pat on the back and some extra pennies thrown your way because you're "doing your job right
Exactly the same argument applies to waiting staff in table service restaurants.
Someone upthread says that she and her DH gets around £1k in tips per month. It's not clear whether that is between them or each, but either way, that's an astonishing amount of money and about the same as the entire wage of someone working full time on NMW.
I can't see how anyone could argue that any level of restaurant service is worth that much more than the service provided by a fast food worker, someone working in retail or a carer, none of whom get a penny in tips.
DP used to work in a fairly high end bike shop and would spend ages helping people buy the right type and size of bike, with custom fit saddles, accessories etc, do all the bike to work paperwork and build the bike, and adjust it after they had ridden it for a few weeks, all of which could be an hour or more directly 'servicing' the customer, who had often spent thousands, all for a few pence an hour over NMW.
They did get given a few beers from customers on very rare occasions, but apart from that, that level of service got not a penny in tips, despite offering a personal direct service.
Still don't see the logic in waiting staff and hairdressers being deserving of often very substantial tips for meeting the basic expectations of their job.