Tethering is not allowed in the T&C's of all the majors for obvious reasons.....
Virgin are not the only PAYG offering auto top up.....
Check this out.....
giffgaff - Unlimited Texts & Unlimited Web with our Free SIM cards I use them with iPhone and everything works on the O2 network but with unlimited data......Have a read through the community forums before making a decision...There are no call centres all the problems are sorted on the forums by the community and 'Agents' on line.