From April 2015 the UK Government introduced the Immigration Health Surcharge for all temporary non EEA migrants coming to the UK for more than 6 months. This entitles you to access the NHS on the same terms as a permanent UK resident. If you are not covered by the IHS then you will be treated as an overseas visitor.
If you need medical treatment while you are in the UK, you will be treated as an overseas visitor. This means that you would be entitled to obtain health care in an emergency from a General Practitioner (GP) who may charge you for the consultation.
If you are living in the UK for a year or more, you may wish to register with a GP but this may only be appropriate if you are staying in the same area for the duration of your attachments. In addition, if you need hospital treatment, you will be treated as an overseas visitor and may be charged for your treatment. You are required to have your own medical insurance to cover this.