A UK trademark only protects you in the UK, which consists of England, Scotland, Wales and Northern Ireland. There is no protection outside of the UK. A UK trademark is filed in the UK Intellectual Property Office (UKIPO).
A trademark filed in the EU Intellectual Property Office (EUIPO) is called an EU trademark (formerly known as a CTM (Community) trademark).
EU trademark protects you in the 28-member countries of the EU making it a very strong trademark. Current EU countries are: Austria, Belgium, Bulgaria, Croatia, Republic of Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden and the UK.
To open in a new tab, click: What is the difference between a UK and EU trademark?