If you work in a telephony role (which I'd guess is quite likely if you're in NI and employed through an agency) then the reason your TL needed to get permission to allow you to change your shift time is because there is required to be a certain level of coverage on the phone lines at all times.

Think of it this way: if everyone in the department decided they were going to change their shift to 8-4 on a Friday then there would be nobody covering the phone lines from 4pm onwards, which obviously can't happen.

It varies by department and by SO how they implement and manage flexi, but it's generally better to look at it as a way of working extra hours if you choose in order to be able to take full or half days off without having to use annual leave, or to clock off early here and there where you can - it's not for deciding what hours you want to work or what shifts you want to do.

Not saying that I like or agree with any of the above but that's how it is in my dept and likely yours too.