to do that virgin need to transfer your mx records.
Because your old ISP is under no obligation ... they will do it when they're ready +10 days (to get you back)
if it's just your personal mail (not business) ... consider a Gmail account ... or one of the others .... that way you change ISP ... you still have your e-mail
as the offers of cheap or free broadband increase ppl will change their ISPs much more often ... so a "static" mail account makes sence.
Google have some pretty nifty "extras" like online office so are worth a look