You have completed the BIFM (Basic Instrument Flight Module) in the CPL, so it is indeed 45 hours for a multi-engine IR.
A minimum of 15 hours must be in the aircraft. It cannot be 10.
All EASA states should require 15, so I would suggest the Belgian based school is wrong.
Once you have your UK CPL, you can add a Rating to it in any other EASA state. A popular one was the Spanish IR on the UK CPL.