belgium pension tax

A Belgian state old age pension is not taxed at source in the way that a UK pension is. You will be liable to pay tax on it in Belgium but, if it is below the tax threshold, there will be no liability. (I have a Belgian state old age pension and that is my situation.)
Presumably you will have to declare it in the UK but, again, it may be below the UK tax threshold so there will be no liability.

A Belgian state (and any other) old age pension paid to a Belgian (or anyone else) living in the UK will be taxed in the UK and not in Belgium. Kasseistamper's situation is not comparable because he lives in Belgium (sorry Mr De Wever, Flanders).

The situation may well be different, though, in respect of any pension which relates to employment by the Belgian state. So long as you remain a Belgian citizen (even if, forbid, you also become a UK citizen), such a pension will be taxed in Belgium under the terms of the double-taxation treaty. If that is likely to be the case, I suggest that you seek an appointment with your current Belgian tax inspector before you go as that will ensure that proper liaison is set up between the Belgian and British tax authorities.

I should have added that it also incorrect that a UK STATE pension is taxed at source but other pensions paid from UK-based sources are. Those over the state retirement age who receive both have the tax code which is applied to the non-state pension adjusted to take account of the money they receive from the state but it is always paid gross, not net of tax; the non-state pension is, of course, lower as a result because of the greater tax deduction.