Since I do not have evidence for what I had experience. Here are some advice on the net:
https://www.independent.co.uk/money/spend-save/tips-on-tippi...
https://www.lhaa.co.uk/blog/how-much-to-tip-a-hairdresser#:~....
Although it is always worded "if-you-want-to" or "if-you-can-afford". I feel this is a bit ambiguous.