I agree with a couple of the others in fervently hoping that you do not intend to go from Heathrow to Paddington on the Heathrow Express and then on to St Pancras from there. That is so unnecessarily expensive for 25 of you that it makes the eyes water, and it involves having to change from one mode of transport to another. The Tube (Piccadilly Line) will take you direct from Heathrow to St Pancras. It's the start of the line so you should all get seats.
If we've got this wrong and you're actually arriving at Paddington from another town in the UK, five black cabs would take you to St Pancras, as five people can get into each cab.
Edited: 6:48 am, July 09, 2013