score:8
I just ask it to book a ticket from Heathrow to Canterbury. Any terminal will work but it's cheapest and fastest from Terminals 2&3, which is where the main bus station is. If you're arriving at some other terminal, you can get to T2&3 by Underground from T5 or bus from T4, both free.
Go to nationalexpress.com/en Enter "LHR T 2/3" as the departure point, and "Canterbury Bus Station" as the destination. Select a date and time. Hit "Find My Journey." The site will display the schedule and prices.