Just an update: So I booked a carhire from Simply Airports to pick us up from St. Pancras. We were picked up from the taxi drop off area. Took about 1 hour to get to LHR using what seemed to be a zig-zag route, but I imagine it was the fastest way there. Cost about 34GBP. Showed up at T1 and was able to get folks on the flight just fine. Worked out perfectly; thanks for the input everyone! I do appreciate it and really loved my time in London!