How long have you been out ok the UK? And where are you returning to the UK from?
Those factors, as well as the type of care you'll receive will effect your entitlements. In April 2015, laws on British expats using the NHS changed. Those from outside the EU, will be required to pay 150 percent of the cost of hospitals bills but they will receive free GP consultations and emergency treatment.
Take a look at the Healthcare in the UK for more info.