Yes, the United Kingdom is eligible for Horizon Europe, as it has become a third country associated to Horizon Europe. Therefore, legal entities from the United Kingdom can participate under equivalent conditions as legal entities from the EU Member States, unless specific limitations or conditions are laid down in the work programme and/or call/topic text. The United Kingdom will participate in all parts of the Horizon Europe programme with the only exception of the EIC Fund (which is part of the EIC Accelerator of Horizon Europe that provides investment through equity or other repayable form).
The list of participating Countries in Horizon Europe is available form the European Commission website and has been published on the 17th of June 2021. It lists all other countries eligible for Horizon Europe.