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.
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.
Just a few things that can affect Wi-Fi in the average home …
Mirrors, furniture, fish tanks, 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.
is the hub replacement part of the upgrade if so everything will be default - even if you have had it a while you want to make sure that the 2 bands are split and smart wifi is turned off
post some info from the hub to be sure its levels are ok
log into hub3 - 192.168.0.1 [or 192.168.100.1 if in modem mode] - DON'T SIGN IN, click - click router status [centre of screen] - and post the downstream and upstream figures - to see those press on the grey buttons/boxes labelled - upstream - downstream
i accept your point that you got 70 to 100 whilst on 200 - what did you hope for or what were you were told would happen by upgrading to 500 - the best you would get is the same - the speed is not determined by getting 500 but by your devices