What kind of plane are you talking about?
At some point in time, it would be potentially better to register it under OO. Don't forget that you will need a UK inspector at east once a year.
Anyway, if you are willing to operate a non OO registered experimental (or a G permit to fly) in Belgium, you will need to apply for a permission every year.
Aanvraagformulier - Formulieren - FOD Mobiliteit
or
http://www.mobilit.belgium.be/fr/bin...467-207147.doc
Also you need to pay 96EUR a year for this authorisation, be limited to 30 flying days during this year, submit a flight plan for every flight (also send it to a specific e-mail address) and send at the end of the year a report with the trips that you made.