and what's more once they've given you a start clearance they'll normally launch you off regardless of whether you are more than 5 minutes early or greater than 10 minutes late.
I would hope that they are not 'launched off regardless' but that any departure outside slot tolerance is either co-ordinated with CFMU or a local Flow Management Position, or is part of an agreed discretion scenario. The UK CAA have stated that a slot is part of a clearance and therefore has to be complied with.
The result of not complying is to cause a potential overload somewhere down the line. This results in a reduction of safety and the possibility of a more serious incident occurring because ATC are unable to cope with the traffic levels presented to them. The subsequent investigation will always then lead back to the ATC unit who didn't comply.