How to exit the Belgian system please?

Ask the commune. You'll have to ask them at some point - they probably have a checklist you can start from now.

Just leave.... do you really think any Belgians living in the Uk would give a toss about leaving the UK. We don't have any such nonsense in the UK so if I was you I'd Just pack and go... it's not your problems once you have gone.

Are you employed or self-employed?
If you're an independent you have to close your 'business' (terminate your VAT number); leave your mutuelle; and quit the commune.
The commune might be the first port of call. Note that your mutuelle charges you quarterly for every quarter - or part thereof - you're in the system. So if you leave in July 3 you'll still pay for Q3 2013.
You could just do a runner as Gecko suggests but there's always a chance that you'll receive a bill at some point if they track you down in the UK. Not sure how easy that would be for them but it's not like you're moving somewhere far away beyond the EU.

Nice one Gecko. Just stick your head in the sand and potentially miss out on a huge tax rebate. Yeah - you do that - just go.

be sure to de-register at the commune so that you will not be liable for any future taxes. As long as you remain on the books, you are liable for taxes.

@J .... you make a big assumption that the dperting person pays tax. I don't :-)

It's a typical Belgian standoff- one Belg says you have to deregister to get a tax rebate the other says you have to deregsiter to avoid having to pay more tax... as with most things it's just random.

@Gecko
Given that you clearly don't know what you're talking about, you should do us all a favor and STFU.