So if you join BA after next month you've been given a pay cut of around £1000 per month for the first 3 months
Not even close. Long haul Flying Pay Allowqnce was/is just over £600 per month so after tax under the new regime you'll lose approx £360 per month. BA isn't keen on having pilots sitting around so you'll almost certainly start your line training within two months of your start date. So rather than lose £3000 you'll actually lose closer to £720 net of tax which you recoup quickly at the new Flying Hour Rate.