That seems about in line with what it costs, I'm afraid. You're paying for 2 days parking (and would at any other London airport).
I would go with pod parking, it's barely harder than any "meet and greet" sort of service (as long as you can walk OK - but if you've managed to walk from the aircraft to the customs exit, it's probably less walking to your car from there than already done from the aircraft door). I have never had to wait more than a minute or so for a pod, and usually there's one waiting for you.