The problem with Wi-Fi is that it can be brilliant at breakfast time and useless by lunchtime and you haven't changed anything, BUT, something, somewhere has changed, then you can spend hours and even days trying to find a fix for something that can be totally out of your control.
If you remove all the internal walls, water, electronic and electrical devices, metallic pipes and conduits and wrap the whole building in a thick lead shield you will have a good chance of getting decent Wi-Fi from an ISP Hub.
Just a few things that can affect Wi-Fi in the average home …
Mirrors, furniture, fish tanks, people, radiators, walls (some more than others), cordless phones, baby monitors, microwave ovens, fridges & freezers, TVs, next doors Wi-Fi and Wi-Fi equipment, DAB transmitters, the list is endless.
Many people have accepted that the VM Wi-Fi is poor and have purchased third party equipment that is much, much more powerful than the VM supplied kit and once set up have wondered why they waited for so long.