The United Kingdom is a party to the Hague Service Convention, and serving U.S. process there is comparatively straightforward. For England and Wales, formal requests are handled by the Foreign Process Section at the Royal Courts of Justice in London, under the Senior Master of the King's Bench Division; Scotland and Northern Ireland have their own authorities. Documents in English need no translation.
You don't have to use the Central Authority. The UK has not objected to direct service, and the UK government confirmed in 1980 that a competent person such as a solicitor may serve documents directly. In practice, most service in the UK is carried out in person by a process server acting on a solicitor's instructions, which is much faster. The Central Authority route is worth considering when you may need to enforce a judgment in the UK.