You can receive emergency treatment for up to 14 days if you become unwell whilst away from your permanent residency.
If you are going to be in the area for more than 24 hours but less than 3 months you can register as a temporary resident at a local GP practice.
Practices do not have to accept you as a temporary patient although they do have an obligation to offer emergency treatment. You cannot register as a temporary/emergency patient at a practice in the town or area where you are already registered.
For more information, please visit NHS Choices website