The best way to describe the dress code is smart casual. You may find, that many people define smart casual differently in the UK. But as long as you are comfortable and your clothes are clean and neat, there are no issues. Smart casual can be defined as:
Neutral closed toe shoes, long trousers and no exposed shoulders. Trainers (runners) are acceptable due to the long periods spent on your feet.

It’s always best to check with the individual school of what fits their dress code as you may find different conditions apply.