In the UK there has so far only been possible to register third-level domain names (domain.co.uk), which in the future may change. Nominet is launching an investigation into a new service which, if passed, could mean big changes for .UK.
The new service, called direct.uk, targeting companies and means that you then may be able to register domain names directly under. UK (likely a company in the UK will be a requirement). Name ISP will return as soon as new information is available.