Bloggs,
Only those departing from a European Airport could/will be delayed.
There is NO vectoring, extra track miles, speed control enroute for flights overflying European Airspace into LHR, the delays (if required) are introduced by London control on first contact, along with the STAR. There are 4 gate posts inbound and holding if required will generally be at one of these gate posts. from the UK AIP "no delay" means less than 10mins holding.
Extra slots are available into LHR during the day, especially for VVIP flights. The number of VVIP a/c in LHR for the recent wedding is testament to this.