make sure you discuss the aspects of planning requirements fully with your chosen installer prior to starting any work.
If you want to know more about the benefits of replacement windows here is a guide: Double Glazing Vs Triple Glazing? – The Pros & Cons
Double glazing is also good for lessening condensation in homes. This means some 7.5% savings on your energy bills.
To be frank, there is no straightforward answer to this question because there are too many variables to take into account. you could try www.doubleglazingpriceguide.org.uk