As G-CIVC says above.
There is no special sub fleet for any destination. All 77H and 77G (the two sub types of CX's 77Ws...the former is 4 class and has F, the latter is 3 class and doesn't) frames can go to every city they're rostered to, including LHR. So until the refit is 100pct complete, you can't be sure on the 77W if you'll get a legrest-refit PEY or not. If you absolutely must have a leg rest, then your only choice to guarantee it is fly LGW-HKG on the A359.