UK General visa inquiry

You were detained and removed from Belgium, even though you never 'officially' got to land side. So you would write it down in Section 6.2. The duration of your visit was 1 day.

More importantly, when you get to Section 6.6, "Have you ever been deported, removed or otherwise required to leave any country, including the UK in the last 10 years?"

You should check the "Yes" box and proceed to provide the gory details. For the 'reason' you can write 'Visa annulled'.

I should state Turkey only or Belgium as well and should I send the old passport to the embassy or just a scan from it?

Since your old passport contains important evidence, you should consider sending it with your application. It's not required, but if you have an otherwise clean history sending your old passport can help to prove it.

And will it affect the visa decision or not.

Well, yes... Normally they will worry when someone's history contains a removal; they like it better when people have clean histories and great performance. So the odds are against you initially. To turn things back to your favour, you can write a compellingly sound explanation of the event that mitigates its gravity whilst at the same time showing candour and transparency. There's no requirement to provide an explanation, it's a 'best practices' approach.

You can use the "Remarks" section of the form for your explanation or prepare a cover letter. Needless to say, the rest of your application should be exemplary.