St Pancras station to Canterbury?

Canterbury East and Canterbury West are both about the same distance from the cathedral. It's about a 10-15 minute walk either way.

I'm not sure where you're seeing those exact fares (I'm seeing slightly different ones) but when I search for that journey on http://www.southeasternrailway.co.uk, it shows some advance-purchase fares that don't show up on http://nationalrail.co.uk. If you don't mind being locked into taking a specific train at a specific time, you can buy an advance ticket at a lower fare. If you want the flexibility to take an earlier or later train, you will have to pay more.

I was also told I could get a non-stop train? Don't mind have a booked ticket outbound but would prefer a flexible return ticket!
Thanks,
Jean

"I was also told I could get a non-stop train?"

no - the train will stop at some stations - slower trains will stop at many stations, the fastest trains only stop at a few stations

There are two direct routes from London to Canterbury. The quicker and more expensive route is from St Pancras on HS1. There are also standard, slower services that leave from London Bridge, Charing Cross, Cannon Street etc. Either station will be OK for the cathedral.

Book with southeasternrailway.co.uk, the operator. I would be surprised if you can get to Canterbury for £8.50. It is more expensive from my local station which is closer to London, but rail ticket pricing is a dark art these days.