Hong Kong Kowloon to Guangzhou by Train

Unfortunately, you are not allowed to take a train because the only eligible way to enter Guangdong province, using visa free transit scheme, is by air. You can exit it by different means, but the entrance should be by air only.

To summarize, in order to enjoy 144 hour visa free transit valid for entire Guangdong province (72 hours have been replaced by more generous 144 hours years ago), you will have to fly from Hong Kong to Guangzhou in order to join to your flight from Guangzhou to UK. Those two flights can be totally unrelated and on separate tickets.