Answered by ZUPAN | Jun. 12, 2019 06:56
Your itinerary UK - Guangzhou - Hong Kong, assuming that your flight UK - Guangzhou does not have other stops within mainland China is eligible for 144 hours visa free transit. Please, keep in mind that you have to stay for at least 25 hours within Guangdong province to be allowed to enjoy 144 hours visa free transit (72 hours is replaced with 144 hours).
Anyway, I have to inform you that it is not allowed to take a high speed train from Guangzhou to Hong Kong. You are allowed to take a so called Guangzhou–Kowloon through train departing from Guangzhou East railway station and arriving at Hung Hom railway station in Hong Kong. This train is also just fine because it takes just 2 hours between the two mentioned cities. You HAVE TO book a ticket for that train in advance and you can do it on Guangdong Line Schedule - MTR Intercity Through Train website. You have to travel with exactly that train you have bought your ticket for.