This is a Yellow slug crawling through a wormery that I saw at the London Green Fair recently. I mistakenly called it a Leopard slug at the time, not being particularly familiar with slug species and assuming that this spotted thing must be one. But now I know better having looked it up.
However if you've never seen a video of Leopard slugs mating, I suggest you do so right now – but not if you're eating or otherwise easily repulsed. It's truly bizarre.
We are possibly one of the few couples who often find slugs in our cabbage etc. This is because we go to a place which does not use sprays and where we get them pulled from the ground. Our policy is to release the found slugs into the “wild”. Whether, after your post we will try breeding them is an interesting question. It has to be better than watching most of the stuff on TV at present. We will look out for leapard slugs and let you know.
Looks ugly 😀