Melanotan II is only available as an imported medicine and any such importation is illegal under Section 7(3) of the Medicines Act 1968 (because the product is unlicensed in the UK).
Section 45(2) of that act states:
"Where any medicinal product . . . is imported in contravention of section 7. . . . of this Act, any person who . . . is in possession of the product . . . knowing or having reasonable cause to suspect that it was so imported shall be guilty of an offence".
So possession of Melatonin II is a criminal offence.
Further, Melanotan II is not licensed in any country in the world and is thus likely to breach import rules if you take it on holiday with you. So you'd be risking deportation and/or a fine and/or imprisonment (and possibly, in the case of some far eastern countries, the death penalty) for having it in your luggage.
>>>please no answers about the risks
Tough, 'cos you're going to get one anyway! If you're prepared to risk your health (and possibly your life) for the sake of getting a tan you must be a total fool (and an extremely vain one at that). Kindly remember that when you're in agony, dying of cancer, in a few years time!