i have extracted this from the visitBritain website
Medical informationMedical insurance
Bringing medicine into the UK
If you want to bring medicine into the UK, first check that it is licensed for use. Always carry medicines in a correctly labelled container as issued by the pharmacist. Otherwise, bring a letter from your doctor or a personal health record card giving details of the drug prescribed, in case it is queried by customs or you require additional supplies. Remember that some medicines available over-the-counter in other countries may be controlled in Britain, and vice versa.
For further information please contact HM Customs and Excise Advice Centre, Tel: +44 (0)20 8929 0152