London Underground does represent good value to me.

I can get from my flat 350m from Pimlico tube station to T5 in about an hour. It costs GBP 3 each way when using an Oyster card and travelling off peak.

If I take the HEX I have to get to Paddington, which involves either taking the tube or a taxi. If I take the tube then I might as well have gone all the way to LHR on the Piccadilly Line. If I take a cab and then go STD on the HEX it will probably cost GBP 30-40 each way.

I have trips lined up to HAM in CE, DXB in CW and SIN in F. I will be very happy to take the tube in each case!