When you first arrived in the UK, you may have experienced culture shock. New surroundings, a new environment, new traditions, etc. can leave you feeling disoriented and isolated.
The same is true when you return to your home country – you can experience reverse culture shock. After spending anywhere up to 3 or 4 years in the UK, you have become accustomed to English ways, English traditions, your new expatriate life, in ways you may not even realize.
The degree and severity of this shock will also depend on what country you are from. The USA and many parts of Europe have similar cultures to the UK and so culture shock may not be as bad as other countries.