No. This is a waste of one of your five UCAS choices as we are flexible.
You can change your mind about your subject choices when you get here (with the one exception that you cannot transfer into Chemistry).
So, in the example listed, you should list your subjects as Maths, Physics and Psychology and if you want to choose between Physics and Psychology, you can do that when you arrive.
The order in which you list the subjects does matter though so give it some thought.
You also need to satisfy the subject specific requirements if you want to change, see Entry Requirements, so in the above example to study:
- Maths and Physics- the offer require subjects of Mathematics, Further Mathematics and Physics with a specified A*.
- Maths and Psychology- the offer requires subjects of Mathematics, Further Mathematics and another A-level.