hmmm... ok...
well usual culprits are
loss of fob sync, or non working fob.
low battery
faulty door lock / boot bonnet switch
failure of the A5S alarm subsytem
Other wiring faults
If you have a look at the owners manual https://www.the-t-bar.com/community/downloads/download/10-owners-manuals/26-mgf-owners-manual
it describes the process for fob resync, and also using your key to shut down the immobiliser via the drivers door - if you know what your EKA is.
If these dont work, then diagnosis becomes a bit more complex.
There is more info in the workshop manual
https://www.the-t-bar.com/community/downloads/download/8-technical-manuals/18-mgtf-workshop-manual
and the wiring diagrams sheets https://www.the-t-bar.com/community/downloads/download/5-electrical-diagrams/29-mg-tf-electrical-diagrams
Do you have a multi-meter and dow you feel confident about using it to check wiring and voltages?
If so, we may be able to narrow down cause.
If not, where abouts are yiu? A local forum member may be able to help...
And finally, if you havent done so already, pull the connectors off you horn(s), your neighbours will thank you for this while experimenting!