You can get about 6 free sessions with a therapist on the NHS but there is usually a long waiting list and the sessions are often not enough.
However. Go to your GP and tell him/her that you wish to be put on the list for help. As you have had involvement with your GP over this, then that's good and may help you be prioritised up the list.
The GP may well refer you to your local mental health unit first for an assessment to see what sort of counselling will help you. Take all the help they can offer.
Privately, well, you can probably find a counsellor locally (go to the BACP list and find one in your area) to help. They charge anywhere between 30 to 70 pounds an hour but you can choose how often you go (say 30 pounds once a fortnight). Some do telephone coun selling or even counselling by email.
Whereabouts in the country are you?
Those are just practical tips.
On other fronts: Do you have any siblings and if so, what is their relationship like with your mother? Could you confide in them? Possibly not - for many very good reasons. It is just a thought. Crap mothers tend to divide and rule. Is there another underdog among your siblings you could talk to?
Or...Are you an only child? In which case, there are resources for only children that will be worth googling and investigating.
It is great that you have finally started to think that you are not going to take a mother's issues/crap anymore - and yes it is often when you have dcs of your own and start re-evaluating your own childhood.
It's good that she is 100 miles away. What does your dh/dp think or feel about your situation re your mother? I really hope he is supportive.
It is not an overnight thing - going NC I mean - and it can take a couple of years to process. And there will be contact and guilt trips all the time.
But it is eminently do-able and after all, you are doing it for your children.
My mother set eyes for all of 3 hours on my step-girls (marriage over now) and said of one "she is docile" and of the other "she is trouble".
This is not parenting, or grandparenting. This is worth moving to a small island off Japan for.
Do it girl. Power to your elbow.
xxx