I have no qualms about leaving my car there. I have left mine multiple days there before and never had an issue. It’s full of high end BMW, jags, mercs etc and I have never seen one broken into. Just don’t leave your sat nav out.
I’m actually leaving it there tomorrow night as off down in the morning. I’ll let you know if anything happens.
I have tried all sorts of drive/train combinations over the last 5 years and if you’re coming from the north Watford junction is the best as its so close to the m1 and no m25 involved. If you’re going for multiple days then the qpark/purple parking on the bath road (heathrow) is superb as they drive you and pick you up from the tube station at heathrow and it is very secure. Very quick too