A new service started this month between Heathrow and Guildford. The RA2 is run by First bus who also operate the RA1 from Reading.
It's £10 single on the coach but £9 if you buy online. I only came upon it by chance after landing when I checked the Citymapper app and thought I'd give it a go to avoid the PM rush hour crush trains with my suitcase. I forgot about the slow moving M25 but still got my connecting train at Guildford. Heading towards London was no problem on a near empty Southern service.
https://www.firstgroup.com/railairIncidentally if you book online the railcard price is the same as the normal adult fare and it appears that you have to book at least one day in advance and print your ticket.