As you know the UK is a large place with many, many places of interest.
You haven't given any indication of your interests, whether travelling with children, or timescales, so difficult to make any meaningful suggestions. A bit like asking for ideas when travelling to the U.S. 🙂
I live in the UK but would struggle with suggestions based on what you have told us.
You can look at the Visit Scotland and Visit England website which will give you lots of ideas or attractions and places of interest.