How long after death can i apply for a copy of a Death Certificate in UK?

Unfortunately it will probably be out of all of your hands. The best thing when the time comes is to appoint a solicitor to sort out probate. No one can take anything legally until that is sorted. If she never made a will, you will not be able to carrry out her wishes as it hasn't been legally written down. See a solicitor now to find out your position on the joint accounts. You may find that her funeral expenses will have to come out of them.