I'm rostered 21 days. Crew can do 900 block hrs a year
An employer must ensure that crew do not work more than 2,000 hours in any 12 month period, of which total flying time must not exceed 900 hours. This is not a BA limit, it is a legal limit, the only BA must do is ensure that you do not exceed 900hrs in the preceding 12 months and that's it.
75% crew should logically do 75% of that which would be 675 hrs
So you're suggesting that BA reduce your legal limit pro-rata??
Surely it is your decision to work part-time, good luck to BA if they can roster you up to 900 hours flying in 75% of full time.