The Kiev was laid down in 1969, perhaps intelligence gets wind of this or it happens 18 months earlier.
Archibald, the Sea Harrier is a baby compared to the Phantom and Buccaneer, it would be a good compliment these planes as a backup but using them as replacement is a step backwards. In addition the Sea Harrier fleet was tiny even compared to the winding down fleet of the FAA; The FAA handed over to the RAF about 40 Phantoms and 50 Buccaneer between 1970-78, the Sea Harrier order was 24 in 1974 and increased to 34 in 1978.