“We just want a roof over our heads”
A roof only marquee or awning will provide this. Perhaps in England these are most appropriate in the hottest months of July & August.
It is hard to effectively heat an open sided area so a roof only marquee may not be much use outside the summer months, though in a lovely sheltered garden they will maximize the views for your guests.
To be safest though, why not have a marquee with as many transparent walls, windows or French doors as possible. This way you can open and close sides as required, and enjoy the garden whatever the weather.