Prior to the Comminications Act 2003 any company operating in the UK had to do so under the appropriate telecommunications licence. The licensing regime has been replaced by a general authorisation regime with the General Conditions of Entitlement. Compliance with the relevant General Conditions is thus a legal requirement for any Communications Provider operating in the UK.
You can find a current version of the General Conditions by clicking here.
Following the adoption of the revised European Framework Directive in Europe in November 2009, Ofcom in the UK is adding changes to the General Conditions of Entitlement to ensure UK compliance. The changes can be found here