I is lowest J revenue bucket so CX has plenty of seats left.
Redemption seats don't show in EF.
Straight redemption seats can be seen on cathaypacific.com but none currently available.
Upgrade seats cannot be directly seen either in EF or cathaypacific.com.
Have to gamble on which flights will CX release J seats.
I'm no expert on LHR-HKG fares but since all flights are I9 (meaning the J seats aren't selling well - they will be making some J seats available for upgrade first and straight redemption later) I'll use E (lowest PE bucket) and K (lowest Y bucket) to tie-break - CX250 and CX256 look like your best bets.