I'm guessing your mom doesn't work and she's on benefits. If she's on benefits, encourage her to get a part time job. Maybe I'm wrong, but if I'm correct, then keep reading my reply.
If she goes to work she won't have time to bother you because she will have £600 from her two daughters plus the money she will be getting from work.
She will also get money from universal credit if she works part time. If doesn't want to get a job and keeps insisting on £300 payment every month, you can tell her I'm moving out and this will stop her from bothering you.
She knows if you move out, she will lose out on the £200 she currently getting from you. Try this trick and I promise you it will work.
On the brave side, you can move out and share a flat with a friend. Or since you're on disability allowance, you can get a one bedroom flat since you have priority with your condition.
If you are brave enough, you can stop your mom bothering you by using a support worker from your local council. It's up to you. The decision comes from you but my advice will be, go to your local council and ask for a support worker to help you.
You can also go to the citizens advice bureau and they will be able to assist you in making the right decision and proper professional help.
Don't let your mom take advantage of you. Be brave and stop her from taking advantage of your vulnerable state. Good luck, i wish you the best and do take care.