WiFi is WiFi is WiFi. Anything more than one wall or floor between the WiFi client device and the BT Hub will significantly reduce your coverage.
The BT Hubs are actually pretty decent Wireless LAN devices. It’s very unlikely that you’ll find a significant improvement from a direct replacement, located in the same place.
The signal on any all-in-one modem/router/access point is omni-directional so in free space it’s a big globe of WiFi signal. So if, as most of these devices are, your BT Hub is tucked away in a corner on the ground floor then you’re wasting a big chunk of coverage. Relocating your BT Hub to the middle of the first floor can often yield incredible improvements in coverage. Many homes have phone points in the upstairs bedrooms so it’s an easy thing to try out and is completely free of charge. Even if you have to run a phone extension, it’s a fairly cheap thing to do and relocating the device generally gives the best improvement of all the single device options.
If that doesn’t give you the coverage you’re after you then have the option of a mesh type system (such as BTs disc system) and these work fairly well in many homes.
If you want proper coverage then you need wired remote access points and these require you to run at least one cable from your BT Hub location to (usually) the top of the stairs landing ceiling. These ceiling mounted access points are directional and project a curtain of WLAN coverage down and sideways. Sometimes you need more than one to get good coverage in a really big house and a lot of home owners are highly resistant to drilling holes in walls and ceilings. If you really want GREAT WLAN coverage then ceiling mounted access points are the solution you need.
