I believe you can currently get double BA miles if booked through Holiday Extras (10 BA Miles per £) - I received an email this week.

I actually need parking myself in a few weeks. I'm booking two weeks, and was quoted £170 for the T5 valet parking, and £100 for the closest park and ride service.

For £100 I could also have got a hotel+parking nearby (Park Inn), however my flight is at 4pm, so I don't feel it's necessary.