Please do not patronise MachChub for asking a simple question. As this person is from Singapore, I imagine he / she is similar to others in Far Eastern cultures where they will always boil water before drinking it and believe it is not safe to drink water which isn't boiled first, even in their own country.
England isn't quite like places like India, where it is recommended to drink bottled water, but don't shoot someone down for asking this question still. Someone travelling away from their home country may not be used to the water there, and it can be a concern.
Bottom line: Yes, it is safe unless you may be in a dodgy hotel. But if you are really concerned, buy bottled water. And ignore the silly responses in this post and only pay attention to those with helpful and sensible answers.