I think private health care in the UK varies from hospital to hospital. You may have to contact the hospital to get an idea, but ultimately the cost will depend on the treatment needed.

With cancer it's difficult to predict what treatment will be needed because it will depend how the patient reacts to treatment and how far the cancer has spread. The initial diagnosis may change halfway through treatment, and additional treatments may be needed.

In another thread here, someone said that private cancer treatment in India is very good and less expensive than in UK. You may find that it will be easier to seek treatment there.

Just as an ilustration of cancer treatment costs in France: 1 day in hospital costs around 2,000 Euros and that includes all care, medical exams, lab tests, blood trasfusions, and surgery. If a patient needs to undergo surgery that requires hospitalization, the total cost is calculated by multiplying 2,000 Euros by the number of days in hospital. 

Chemotherapy may cost anything from 5,000 Euros to 30,000 for a short course of a few weeks, and 50,000 to 100,000 for longer course, depending which drugs are used. In France they have laws that make it ilegal to run hospitals for profit, all prices are set by government, but in UK it's different, hospitals can charge whatever they like, so I guess UK will be more expensive than in France.

Hope this is helpful.