As EU-citizen, you can travel to the United Kingdom with your identity card until 30 September 2021. From 1 October 2021, a passport will be mandatory. A visa is not required when travelling to the UK for stays shorter than 6 months. Check here if you require a visa for you stay.
When travelling to Belgium from the UK, you will have to deal with customs for persons and luggage. From 2021, there are different customs rules for carrying goods from the UK than you are now used to. You may also face more extensive customs checks when returning to Belgium from the UK than now. More information about quantities and amounts can be found here.
The European Union also imposes strict rules for travelling with pets. Make sure that your pets are vaccinated if you take them to the United Kingdom because for them too this will be a trip outside the European Union. More information can be found here.
Make sure you inform yourself before your trip by checking the latest news via the website of Foreign Affairs.