gooseman, I know you asked about cargo rides, but thought I'd chime in on the chance you decide to go pax.
United, USAir & Delta are all good to ride with to London. They serve Gatwick & Heathrow, don't think any of them go to STN. UAL has several departures out of ORD each afternoon to LRH.
If you are going on the pax carriers, it is a good idea to pre-pay departure taxes at a customer service center. They will give you a receipt and IF you are asked for the fees then you can present it. Sometimes they don't ask for them. If they do ask and you are already at the gate without having paid, you may end up missing the flight as you backtrack to the appropriate office, stand in line, etc (especially at LRH).
Heathrow Express is an easy way to connect to the city and rail system from LRH. It's not cheap (15 pounds OW when I rode), but aircrew can get a discount, I think by paying at a ticket window (they can't give the discount when you pay on board).
Hope you make it and have a great time! London is the :bomb: !!!