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My guess is you'd be charged for something like a public order offence or whatever they would charge you with for walking around displaying an illegal image in any other way. You would, I imagine, be met with a court order to keep the tattoo covered in public, and would be punished for breach of said court order if you failed to do so.
If it was on your arse or somewhere else you kept covered, nothing would happen, though they'd probably want a word with the tattoo artist who did it.