You just reminded me of a thought I had of tattooing tiny numbers from 1-200 1/4 inch apart all over my belly to make sure I rotate injection sites!
Tattoos are not available on the NHS (but their removal is). There are thin plastic templates available, free of charge, for just this issue. The one I have covers the belly and the thighs. It is simply a thin sheet of plastic with 20mm holes in it. I had to lean on the specialist diabetic nurse pretty hard before she would even admit to having seen them, but eventually she produced one from her desk drawer - but that is the NHS for you.