Weekly HEL-LHR are ca 100 legs +12 longhauls (min 24 legs) brings you dangerously close to 150 legs, that is Finnair Lumo. Starting from scratch, on your way up, you'd get 4 s/h upg vouchers (for hitting Gold) + 2 lh & 4 sh more (for hitting Plat) + 4 lh & 8 sh (for hitting Lumo) AND 1+2 giveaway AY Gold (OWS) cards + a healthy points balance to use for award flights or to upgrade (all revenue tix are upgradable with AY). Once the first year is over, you'll get the Lumo allotment (4lh + 8sh + 2 Gold cards) the next year. Excellent upgrade availability.

100 HEL-LHR on BAEC would be some 4000TP + whatever the lh's would bring in (another 1000?), getting you into the higher echelons of the BAEC programme, I forgot what they are since jumping the BAEC ship a few years ago.

Mileage, Avios, AY TP earnings depend a LOT on the marketing airline. Best case scenario on AY - 225% (paid C for AYPlat), same flight on BAEC would earn 125% (?) Similar results when reversed.

PS - if choosing your flights carefully, you can have most of your HEL-LHR flights in the front cabin of the A359! Obviously AY Y flights, upgraded with AY vouchers/points