Aus to HKG with CX is on the A330 which has the PE cabin in front of economy and is served by the economy flight attendants - service is a bit patchy as a result. (HKG to LHR will likely be the 777 which I havent flown in PE as yet, will try later this month to the US).
Catering is economy plus a little more - you get a choice of 2 dishes from the business menu as the main, while the second meal is same as economy. Same drinks menu as economy.
Seat is an extra 6 inches of legroom, a little more width and recline than economy in a smaller cabin (feels quieter and a little more private than economy). Cabin layout is 2-4-2 so you have to watch out for the middle seats as you'll be climbing over people - if 2 together best to go the pairs on the sides.
I find Cathay pretty comparable to Qantas PE although Qantas has slightly better seats (the recline feels deeper), not enough to spend more on flying QF though.
CX service is typically superior to Qantas also - more staff per passenger who work harder in my experience