tax situation if i leave belgium

They won't tax you on any earnings you have had in Belgium prior to the move as they will have been subject to the Belgian tax regime. You will then pay UK tax on your income (pensions, investments, etc) that you receive there.

1) make sure that you file a tax return in Belgium as you will probably be due a rebate and;
2) you will become tax resident when you arrive in the U.K. Anything you earn before you arrive is irrelevant.

Moving is actually significantly to your advantage as you will be able to use two personal tax allowances in one calendar year.

Best time to do this is end of July, as you get your full tax allowance in both countries over 8 months.