Thank you for your question. I am not familiar with how the NHS works or able to give you a referral. From my perspective here in the United States, we occasion come across the same situation where a patient is not able to lay completely flat.
If she is kyphotic (severe forward curvature of the spine), it is possible to position pillows and towels under her neck and back to support her. The bed can be reposition as well. So long as the head and eyes are horizontal to the ground, the surgery can be done.
Cataract surgery takes about 30-45 minutes. This extra positioning does add a slight risk to the surgery, so you and your mother needs to understand that before proceeding with the surgery.
I have done cataract surgery patient that are severely osteoporotic. It is not the most optimal situation, but it can be done. Ultimately, it is at the discretion of the operating surgeon to do it or not.
Since I am not familiar with the NHS. In the US, I have heard moorefield eye hospital has quite a reputation. So I would check with them.
Remember to press the accept button. It was a pleasure helping you with your question. Feel free to ask any addtional questions.