I know this doesn't probably help much but I can log on OK to Yahoo on my 5800 (edit - it's on O2 though I can do it through my wireless router)
Couple of possibles:
1) Your login is case sensitive so is your phone capitalising the first letter of your username (not sure this makes much difference) or password (this will)
2) You do have images enabled as the login uses a picture as the bit you need to "click" - if you have images disabled you won't be able to see it
3) Try logging into another Yahoo service other than mail - eg My Yahoo then getting to the mail that way
Hope this helps (though not sure it will)...
(edit 4 - if you think its your network provider, log in through a wireless network - if you can do it through this but not Yahoo then there is an issue with Virgin - can't see why there would be but given up trying to understand the world of network providers

!)