In the UK, both "toilet" and "loo" are commonly used to refer to the restroom or the place where one relieves themselves. However, the term "loo" is generally considered to be more informal and colloquial, while "toilet" is considered more neutral and polite.
In formal settings or when speaking to people you are not familiar with, it is generally better to use "toilet" to be more polite. However, among friends, family, or in casual conversations, using "loo" is perfectly acceptable.
Ultimately, it's essential to consider the context and the people you are speaking to when choosing which term to use. Being aware of the situation and using appropriate language is a good practice to ensure politeness and respect.