Does a transfer in Hong Kong qualify as a country of entry as flying Cathay Pacific from London?

Unfortunately, your itinerary is not eligible for any kind of visa free transit.

The basic rule for both 24 and 240 hour visa free transit requires that the immediate points before and after your mainland Chinese visit have to be in two different countries or eligible territories. Eligible territories are Hong Kong, Taiwan and Macau. As you can see, you have Hong Kong as both of your immediate points and the situation of this kind requires a visa obtained in advance.

The only way how to bypass the necessity to obtain a visa would be a solution that will bring you from mainland China to Macau. For example, after your stay in Xi'an you should take a flight not to Hong Kong, but to Shenzhen. When you land in Shenzhen you will transfer to Shekou port (located within Shenzhen area) from where you will take a ferry to Macau. Immediately upon your arrival in Macau, you will take a ferry to Hong Kong. Like this you will be eligible for 240 hour visa free transit.

In order to use this option, you will have to buy your ferry ticket from Shekou port in Shenzhen to Macau through Turbojet ferry website in advance, even before you depart toward mainland China. Also, you will print out that ticket and have that printout with you as a proof that you will be really in a transit. You will be asked to present that ticket to both Hong Kong and to mainland China immigration.

If you do as explained you can avoid the necessity to obtain a visa and you will enjoy 240 hour visa free transit during your stay in mainland China. Now, it will be up to you to decide which of the two options will you choose. In any case, I wish you a pleasant journey.