Give someone your Medical Power of Attorney and also make out a Living Will which includes a DNR - that way if you end up in hospital no-one can mess around with you and the person who has your Medical Power of Attorney will see to it.
Are there such things as MPAs in the UK? I've made my wishes plain to my nearest and dearest but I wouldn't trust any doctor not to start fiddling around with me. And as for giving up parts of my body for organ transplants or medical science - absolutely no way. It's all going with me.
I wouldn't worry about long life. You're inevitably going to get septicaemia or hepatitis from dodgy tattooing and die a hideous, mewling death.
If that was realistic then I'd worry
I must apologise to you Quackers. I opened a Tattoo thread, without bothering to check first if there already was one.