Originally Posted by price123

Prompt are fine - a number of people used them for the London Do in 2012. You generally get a Merc E, so better than most of the minicab services available at the £40 mark.
Taxi to Paddington is around £8. Single on the HEX is £21pp, so you're at £50 already. Convenience is lower with taxi+HEX but you have a better idea of how long it will take (usually).