I have done a search but just want to ensure this is going to work before I go ahead and buy the Deco XE75 Pro system. 

At present we have use the BT Whole Home mesh network which was nice and easy to set up and works well however we're having full fibre installed in a couple of weeks and the BT system won't allow us to use the new speeds. 

I have a garden office which is around 15m from the house and outside of range of WiFi but we have an Ethernet cable that runs from the BT Home Hub router and plugs into one of the BT discs which gives us both hardwired and Wi-Fi down the bottom of the garden. I really just want to check that it is as simple with the Deco system i.e. I'll simply be unplugging the BT network and sticking in the Deco one? And also checking that for the Deco device that will be in the office and only connected via Ethernet cable to the BT router that won't have issues in terms of getting internet? 

I hope this makes sense, we tried a Netgear Orbi system and it was an absolute shocker to set up and probably overkill for what we needed!!