If by your "login" you mean
[email protected] then if you leave someone.co.uk your "login" will cease to be (unless you come to an arrangement with isp-someone co uk before you leave).
It is to avoid sudden changes in e-mail addresses resulting from ISP changes that some of us register our own domains, which we can point at whichever ISP we happen to be using at the time. if I change ISP (and I have done, several times) most folks don't even know I've changed.