AFAIK there is no limit on the days. Can't remember the cost, as I've been successfully dodging LHR for a while now. It used to be entirely reasonable, like 3 quid per day or so.

In terms of security, well it's probably not guarded during the night but the tube's full of CCTV...there are easier pickings to be had for somebody wanting to break into a car. If I had an one-of-a-kind handbuilt Hispano Suiza, I wouldn't be taking it there, but then again I wouldn't be taking it to the LHR carparks either.