I very much doubt that a 93 yr old will qualify for a mortgage. She should join her local Credit Union whose staff will visit her at home. After she has paid an initial loan of £100 to join up, which is returnable on leaving, she may then borrow, at very low interest, up to three times the amount that she has in her savings bit. So if she transferred her £300 from her bank, she could borrow about £800/ £900 and pay it back over time. Her £300 would gain gross interest which is better than any bank current account. I assume that she is receiving her state pension, plus a works one, which are added together, as her total income. This is a nasty trap awaiting private pension people.
Of course, she should be getting a higher rate of state pension and automatically qualifies for winter fuel allowances. Her poll tax should also be at a lower rate. In addition, she may apply to her bank for a rebate of any interest deducted from her current account. You gotta ask them every year about the End of March. It might be backdated, depending on the Revenue, who usually allow four years.