There will always be genuine special offers out there so perhaps set aside a month or so beforehand to keep a close eye on these, www.localglazingprices.co.uk/how-much-are-upvc-bifold-windows, before making your final decision.
Local Double Glazing Prices
search online for double glazed windows prices and given the vast network of websites & window installers just in the UK, this could take a lot of time & effort to find a reputable company with good average prices.