So we arrived in Heathrow Terminal 3, waited about 10 minutes for the bus and arrived at the Europcar depot about 20 minutes after departing the terminal. Having checked in online I received my keys almost straight away and within an hour of landing was on the M25. So overall was quite happy that aspect, considering the Europcar reviews.
As for the vehicle, I received a 2018 Mercedes C220d Estate/Station Wagon in Black. Automatic, with sat nav, leather seats, self parking, auto braking etc. 5000 miles on it. I didn't notice how many of these they had, but I'll definitely be booking a PDAR Mercedes C Class again from Europcar.