I overheard somebody explaining that Belgium and GB have aggreements on taxes.
Apperently your english pay, will be taxed as such, and not anymore in Belgium.
If you have an income in Belgium too, then the English part is added to calculate your tax percentage, but then ONLY used on the Belgian part of your income. The English part is thus not taxed twice.
They even said something about being able to reclaim most of your English taxes after two years, if you do not live there... but I have no clue how or what.
Hope this helps a bit: I know nothing about pension, social security etc.