Salaam I want to open a business lending money to pupil ?
If I charge a fixed price on the amount is it OK? For example £1000 I charge £100 so return to me is £1100 and doesn't matter if its for 5 days or 100 days etc

Before you even consider the religious angle, there are legal requirements that are far more important.
In the UK you require a Consumer Credit Licence otherwise you are simply an illegal loan shark..

Salaam I want to open a business lending money to pupil ?
If I charge a fixed price on the amount is it OK? For example £1000 I charge £100 so return to me is £1100 and doesn't matter if its for 5 days or 100 days etc
What is on the religious angle?

Assalamu Alaikum.

It is Haram to charge Interest.

You can call it a fee, or whatever you want, but there are no exceptions. You cannot expect repayment for more than what was loaned.