First thing would be to check why your Sky is so slow. Are you sure you haven't got WiFi problems?
EDIT: Just to explain a little more, the Talk Talk 150 service is delivered over your existing FTTC cabling using g.fast so if they think they can deliver 150 over the cabling that Sky is managing 15 on, then perhaps the issue isn't your connection but your wifi.
Can you plug a laptop in to your router using an ethernet cable and see what speeds you get?
First thing would be to check why your Sky is so slow. Are you sure you haven't got WiFi problems?
EDIT: Just to explain a little more, the Talk Talk 150 service is delivered over your existing FTTC cabling using g.fast so if they think they can deliver 150 over the cabling that Sky is managing 15 on, then perhaps the issue isn't your connection but your wifi.
Can you plug a laptop in to your router using an ethernet cable and see what speeds you get?