Well, after hunting for a better deal all day it turns out that's just how much it costs. I called the only 2 airlines that will ship pets from SFO to LHR (BA & Virgin) and BA requires a pet relocation agent, hence the $5100.
If I book through Virgin I don't need an agent, but the airfare alone is $3260, plus approximately $650 upon arrival at Heathrow. Plus, I've got to get my USDA clearance prior to departure, vet visits for both, which will be another $300. Then there is bribing my in-Laws in San Francisco to take them to the vet, and the USDA, and the airport, all without missing anything. Then, there is renting a car in the UK to pick them up from Heathrow and drive them to my new home here in London. Grand-total (minus the headaches and time lost) $4210.
Ouch. I guess I'll pay the extra $900 for the time saved. Plus I know they'll be safe.
*sigh* The things we'll do for love!