Hey,
Sounds like a lot going on!
Here’s an extremely short guide -
So tattoos are not considered as gbh if consented to and the client is over 18. Yours is 17 therefor under the legal age so his “enthusiastic consent” is disregarded and this would be classed as Sarah committing an offence against him refer to case Burrell v harmer with very similar problems going on as your question it’s likely Sarah will be committing assault occasioning actual bodily harm towards William and then go into the realms of assault ar and mr as well as ABH AR and MR.
This is extremely brief and not all the info you need but it may good enough to make a plan
I would type the long answer however I’ve just finished 4 essays myself and I’m shattered 🤣 good luck though!!