I assume you work for Yahoo or one of its agents, or at least have a commercial reason for your otherwise pointless and baldy-worded question?<br />
FWIW I'm not telling you.
Sorry - but unfortunately that's what it looked like! :-)
Right, I can't give a definite answer, and have no idea what the most used UK ISP is, but a quick browse through a club address-list shows many yahoo, btinternet, tinyworld, hotmail, aol, gmail; etc., plus an assortment of what seem to be employers' company names.
These all seem to work with the club's own web-site. At least I've heard of no complaints except from me, but that's only because my second-hand PC was set up in a strange way that blocks e-mail links held within web-sites!
There are also quite a lot of what seem to be "own-brand" addresses, along the lines of fred.bloggs[at]fred-bloggs.me.uk style, though these may the addresses of the user's own businesses.
I've no idea what the relative advantages & disadvantages of any of these are, but those above are certainly very widespread.