Have you spoke with the school in Belgium? as far as I'm aware the (uk)CAA wont have anything to do with your test and you wont need to pay them a test fee, the only payment would be to have the rating added to your licence.
I have a UK CPL and did my I/R abroad, it was simply a case of the above, returned to the UK and handed my paperwork to Gatwick wih just the admin fee, although things may have changed.