In our experience, it will be difficult to find a one-stop shop for everything you need and you could potentially end up paying project management fees on top of the labour and materials for the service, or there may be an imbalance in the quality of the different aspects.
It is a sound idea to know who is the manufacturer of your windows, and to see them before purchase. A one stop shop is unlikely to be the manufacturer, and I’m sure there are plenty of NVN users who can vouch against the issues experienced with their loft extension windows/doors that the builder sourced for them.
A good local decorator can work well following the installation of new windows and work around the areas that need attention, adjacent to the new windows.
Please get in touch if you wish to discuss further!
Lori at Ayrton