I also used Cathay Pacific twice, very nice airline.
When you first check in, you will get 2 boarding cards, 1 for each flight.
In Hong Kong, just follow the signs that say "Transfer", usually there is a security check. In the departures area, find a screen and look for your flight number to LHR,
that will tell you which gate to board.
There are maps and the gates are clearly numbered, it is easy to find.
But it is a big airport, so find the gate first, then you know how much time to spare for coffee etc.
There is even free internet terminal, although it might be busy.
As above, if you check in luggage, you will not see it in Hong Kong, Cathay Pacific will just transefer between planes.
You carry your hand luggage of course.
It is easy, you will be fine!
