The Sky is checker will only give you the speed to your router. Everything beyond that is difficult for an ISP to promise to an end user as everyone's home environment is different, therefore they don't.
If you're connected by ethernet, that should give you a proper indication of speed.
Via WiFi being so slow, I would look at the location of the router, the channel it is on and look for anything else that might be interfering.
Beyond that I would either speak to Sky or investigate mesh or homeplugs.
Not a sky broadband subscriber
