I am at present at risk of redundancy. I have been told that some people will have to work their notice and some will get paid their notice. Is this legal?
Yes it is.
It is normal to have to work your notice but the employer can pay you your notice if they want to . Up to the employer not the employee.
If you have another job you can ask to be released early but they don't have to allow it.
It depends on the nature of the job. The people being paid to work their notice may have roles which they need to wrap up, or hand over work to others. The people who are leaving straight away may not need to do that.