Take the following with a pinch of salt - I am feeling disproportionately fiesty today, and am fuming on your behalf.
I'd politely reply that although you appreciate that the firm is facing challenging times, your employment rights are protected by law and that you are not willing to accept either (a) the illegal deductions of earnings caused by the witholding of contractually bound commission earnings, nor (b) the unilateral changes to your terms and conditions of employment.
Then I'd leave it to them to come back to you. I would not raise a grievance at this point because I assume you are still kicking it back and forth. A grievance can be raised at a later date...
I think they will back down when they realise that you cant be pushed over and that they are on the wrong side of the law. It won't do you any favours in Miss Popularity contests at work, but presumably you are disillusioned anyway with them for treating you like shit, cutting your pay and trying to wreck your home life too, so I imagine you are not pinning your career on them long term...I think you need to do enough to protect you now and protect your position whilst you find something else.
You could add that you have been advised that you are entitled to continue to work your contracted hours and will do so; and that - given that the company is breaching employment law - any actions taken by the company to continue to try to force you to change your hours (or to penalise you in any way for not agreeing to unilateral changes) will be taken as being a provocative, discriminatory and vindictive, and will be resisted with the force of an employment tribunal ruling if necessary.
You need to record that you are not willign to go along with this, and show that you are completely resisting this (it will stand you in good stead should this end up at a tribunal) Sending the letter in itself should also protect you to an extent from any action they may take to 'actively' select you for redundancy (so to speak)
Given that you say this is a male dominated industry there are arguments for perhaps also saying something like "Given that there are so very few flexible workers in the firm, I appreciate that you may not be aware of the impact of rulings such as the one you are trying to implement. Forcing people to switch from flexi to non-flexi working has a very disproportionate effect on primary childcarers, which is why it is rightly protected in law".... Now whilst that may read as a bit wishy-washy to most of us, to an employment lawyer that would scream out "STEP AWAY from the lady".