New Anglian mechanism (left). Old, discontinued version (right).
If you have old Anglian windows that can’t be opened it is probably due to the mechanism having broken. This can happen due to wear and tear, shifting in the building, or poor installation when new.
Even jammed windows can be opened. The older mechanism is long since discontinued but with minor modifications the later version mechanism can be made to fit.
How long the newer gearboxes will be available for is uncertain.
Whatever make windows or doors you have, they can normally be repaired or improved. That is significantly cheaper than getting new ones installed.