Not every NHS treatment in England is free of charge. There are, for example, charges for dental treatment and, for non-UK residents, charges for hospital treatments. If you are an overseas visitor to the UK you may be charged for some treatments and, depending on how urgent it is, you will usually have to pay in advance.
Hospital treatment is free to ‘ordinary residents’ of the UK. But if you are visiting the UK – to stay with family, on business, as a tourist, or if you are living here without proper permission – then you are likely to be charged by an NHS hospital for the treatment it gives you. Not paying this charge may have an effect on any future immigration application you make and you risk being turned down.
The information above is from the NHS ‘my health london’ website
If your country has a healthcare agreement with the UK you may not have to pay. Please see the links below for more information.