Note the BA 767s have never had the retrofit winglets fitted (nor ever could do) as they are the lone RR powered 767 operator in the world. Retrofit winglets are increasingly seen on the 767 fleets of AA, DL, UA etc.

Certifying a winglet retrofit is linked to the engine fitted too as each engine type has differing weight and C of G therefore some structural differences to the wing exist per type. This needs to be taken into account with the extra stresses and load paths as adding a winglet can change these.

With BA having been the only RR powered 767 operator in the world (QF did once but even these were ex-BA frames) it was not worth the certification costs for a small potential number.

Footnote: One Chinese airline years ago did buy just 3 x RR powered 767s but these are long gone.