General NHS (National Health Service) treatment is based on being a resident of the UK, and this restricts the free treatment that visitors to the UK can receive.
UK visitors have free access to the following treatments
The decision of whether a UK visitor can access free healthcare is the decision of the hospital giving the treatment, and they may ask for evidence to confirm you are eligible for care.
There is also a list of groups who have access to additional free services; you can view these groups here
Because of these restrictions it is recommended that you get health insurance for your stay in the UK.
For further information about these groups and services for UK visitors visit the NHS website.