Driving a UK car in Belgium...

Where are you primarily and fiscally resident? If you are registered in and are doing work in Belgium, you are highly unlikely to get away with keeping your car UK registered. I know there are grey zones but the Belgian police are highly unlikely to want to hear your excuses and you should be aware that not registering the vehicle here is treated very seriously, as tax evasion no less.

The only people I know of who have got away with taking the piss for decades are a certain family selling English products in Belgium because their cars belong to a UK company.

'I have to keep my car on UK plates'
Why? You give no explanation.
Legally, if you - and your car - are 'living' here for the time being, the car must be registered here.
However the critical thing is insurance. If you are, in fact, living here, are your UK insurers aware of that and happy to continue cover? The fact that you have a 'green card' is irrelevant if it has been issued without your insurers being fully aware of all the facts.
The last thing that you want is to be involved in an accident and your insurers refusing to cover you. You are then facing the prospect of being charged with driving whilst uninsured.
And, BTW, your UK insurers will not refund any premiums!

Kasseistamper,
If you have insuance in any EU state it automatically covers you with the minimum requirement for each state no matter what, this is EU Law. It does not fully cover you so if it is fully comprehensive and the country you are in only requires a minimum of third party then you will not get the additional coverage.
Despite all of that yes if you are a resident here and your car is here for greater than a specified amount of time you will have to register it and pay the ridiculous taxes here.

You are registered with a commune in Belgium, therefore you are resident in Belgium, not the UK where you have a SECOND home. (Incidentally, I hope that you are declaring that second home on your Belgian income tax return as it is potentially taxable in Belgium.)

As a consequence of your registered residence in Belgium you are required to reregister your car in Belgium within 6 months of the date on which you officially became resident in Belgium, and, of course, that will entail Belgian insurance, road tax, etc. plus making changes to the car (headlights for example) to conform with Belgian requirements. If you keep a car in the UK for sole use there you may register that car in the UK (although you would still have to inform your insurers of your Belgian main residence) but you would not be able to drive it in Belgium. (The same, incidentally, would apply the other way round if your principal residence was in the UK.)

The only possible way round this situation is if the car is owned by a UK company, but that company has to actually TRADE in the UK and not just be a shell company created to own the car. (It is amazing how the proportion of Luxembourg-registered cars has dropped dramatically in my area since the fisc started enforcing this rule - five years ago, it was about 1 car in 20, now it is about 1 in 200.)

You are skating on very thin ice by continuing to drive a UK-registered car beyond the six-month period of grace. The local police are bound to spot it sooner or later and take action, and you risk losing the car as they can impound it. The insurance situation, well spelt out by Kasseistamper, is even riskier as the penalties for driving without valid insurance are, rightly, severe, not just from the point of view of fines (or even prison) but because of the effect it will have on subsequent renewal premiums. Incidentally, if your insurance company isn't completely au fait with situation, you will be driving uninsured in the UK as well as Belgium.

Your best bet is probably to buy a car here in Belgium and keep (or sell) your old car in the UK.

Richard, I am afraid, does not understand the situation.

When dealing with your (car) insurers you are REQUIRED to tell them about anything that materially affects that insurance contract and if you don't the insurance is null and void. If you take out car insurance in the UK on the basis of a UK address which you happen to own, or rent, (and perhaps sometimes reside at) and you don't tell the insurers that you actually live (i.e. are registered as living) in Belgium, then you have failed to tell them about a material fact and the insurance contract IS null and void. You may have the piece of paper and pay the premiums, but the paper is worthless as you will find if you come to make a claim. (Note that making a successfully claim under these circumstances is an offence under the Theft Act and will result in a term of imprisonment on conviction.) If you doubt that it is a material fact, just look at the difference between insurance premiums in the UK and those in Belgium.

EU law certainly requires Europe-wide minimum cover in every policy but if you don't have a valid policy, you don't have cover anywhere let alone EU-wide.

@Richard
'and pay the ridiculous taxes here.'
My daughter and I drive identical cars. Hers cost £145 this year in UK - mine a 'ridiculous' €139.13 (Just under £100)

I think the six months rule does not mean you can use a UK car for a couple of months and be free and clear. As far as I am aware, the six months is from when you arrive.

You need to check with your insurance broker in the UK and check your residency status. My broker would insure me for trips lasting up to 60 days outside of the UK so providing I went back and forth in that time, they told me I was covered. As I was on secondment, so was technically still resident in the UK and paid tax there, I left my car on my UK plates. My colleague drove a car which was registered in a family member's name and told me that was perfectly legal, so also didn't change her plates, which were German.