You're going to get a shock if you think you'll be doing 1 or 2 trips a month on a blindline on whichever fleet!
Last time we had variable pay it was around £10 an hour, so even on 900hrs a year it could only be 9k before tax. The money is being kept, just redistributed, so in a normal month take home will increase but will go down in a quiet reserve month/leave/really quiet blind lines. There's no certainty at this stage it will be voted through anyway.