Is Chinese Transit Visa required for Hong Kong to Guangzhou by train then flying to London?

I'm UK resident with UK passport, planning to take direct train on 27th January from Hung Hom to Guangzou East, then subway line 3 or shuttle bus to airport, and have a booked flight ticket to London Heathrow. Can I enjoy the 24-hour free period for the short transit within China?
Would the same free period apply in the opposite direction to the above, if I pre-booked the train ticket for Guangzou East to Hung Hom, for ticket collection at the station, or must I get the ticket posted to me in advance, so I can show it to Chinese Customs when leaving the airport?