End of Tour and Flights

hi all, hoping someone can help me with this question

i haven't booked my flight yet and am wondering how best to co-ordinate it with the end of tour. my tour ends (and starts) in london at "6.30pm" so my questions are:

1. how on time can i expect this to be?

2. using this time as a guide, what's the earliest i could expect to get myself to heathrow airport and by what means of transport?(are the cabs there really that bad?)

3. does the tour do drop offs at the airport on the way to the of royal national?

4. is it even sensible to expect to leave the country the same night as the tour ends or better to pay for a room to get some kind of rest that night and leave the next day?

thanks to anyone for their advice