The Department of Health’s regulations are that anyone who has been living abroad for more than 3 months is no longer automatically entitled to free NHS treatment. This includes people who receive UK state retirement pensions. Whether your father remains exempt from charges depends on the following conditions.

• If a person is a UK state pensioner and lives in the UK for six months or more each year, and not in another EEA (European Economic Area) country for more than six months a year, they are entitled to free NHS hospital treatment whilst living in the UK. To receive this entitlement they must be registered as a resident in the UK.
• If a person is a UK state pension holder and is registered as a resident in another EEA country they must have an E121 form, which provides for entitlement to medical treatment in another EEA country. In this situation they are entitled to free NHS hospital treatment if treatment is needed promptly for a condition that first developed after arriving in the UK. This means that a person cannot have treatment of an illness, including cancer, which has been diagnosed overseas. They will also need to have lived in the UK for at least 10 years continuously at some point in the past.
• If a person lives for more than 3 months of the year outside the UK in a non-EEA country they can receive free NHS hospital treatment if treatment is needed promptly for a condition that first developed after arriving in the UK. You will also need to have lived in the UK for at least 10 years continuously at some point in the past.
• If a person lives anywhere abroad for more than three months, but then return to the UK to live permanently, they will be entitled to free NHS hospital treatment from the day of return to the UK.

Therefore, if your father lives in the UK for more than 6 months each year and is registered as a resident of the UK, or intends to move back to the UK permanently, he may be entitled to free NHS hospital treatment. 

However, if he is not entitled to free care and you want to look into the costs of his having treatment in this country, the charges are fixed by individual NHS Trusts. This means the exact charges and the conditions which apply do vary from place to place. So you would need to enquire at whichever Trust you would like your father to be treated in order to get more precise details of the possible expenses involved.