Yes, I am almost positive it is legal. To be absolutely sure, visit this website and contact someone about it:

But the rules involving importation of animals into the UK are basically because of concerns over rabies and other zoonotic diseases, certain creatures are exempt, that includes reptiles, and I’m pretty sure rodents too.

there are leapards all over the world laying eggs lol, all you have to to is call there customs,
here is a site that can help you

here is a better one

The matter isn’t if it is legal. Ecological consequences might be the real problem. It is going to need a female after all (or a male if it is a girl). What if they lay eggs around destroying some sort of balance. There are examples in the past that indicate such disasters. In a Pacific island for instance, some rats came out of a ship. Now the rare birds in it they are near extinction! No one can stop the rats.