The problem with ventilation is that it lets in the outside air, hopefully circulating it back outside in some fashion. To cope with this you should consider trickle vents on your windows. To find out why they are important, read this article: What are Double Glazed Window Trickle Vents for?
For double glazing, what is the Average cost per Window and where to compare prices? Compare them here: www.compareglazingprices.co.uk