transferring Belgian contributions to UK pension

The system is currently simple, it won't be post-Brexit.

Contact the Belgian pension service and ask them.
Hopefully, you will get about 1/6th of the Belgian state pension from the Belgians.

Oh - and finish the admin process - you'll probably get a tax refund after it.

To get more free information suggest you email your questions to Europe Direct. Email answer takes 3 days.
Just fill in the email form with your questions.
https://europa.eu/european-union/contact/write-to-us_en