You're probably better booking separate. I'm always looking for deals to Orlando (kinda-gf out there) and the cheapest I find generally is.
Book flights with
http://www.charterflights.co.uk/chea...n/usa/orlando/
Book car rental with
www.netflights.com
But do check expedia (& virgin as you've already done) to guage other prices.
Also, DO make sure you have your first night booked and an address of where you are staying because US Customs won't let you into the country if not. Probably a good idea to make sure it is local(ish) to.
Virgin tend to drop their prices about a month before travel and its cheaper to fly on a week day. Their planes from MAN however are the old kind, and are the worse of their transatlantic fleet, its not worth paying extra for them as they are generally worse than cheaper airlines even. If its easy enough to get to a London airport, consider that.
Netflights, Virgin & Expedia all do quidco to.