The speeds that you want, at the distance you want, with the constraints, make this a rather tall order.
To get the 400Mbps speed you want in the garden office would be best managed by running an ethernet cable if that's possible. Otherwise, you are probably looking at either using powerline, or a directional antenna mounted on the outside of the house. To get 400Mbps from a powerline, it'd need to have a path that doesn't cross from one ring to another via house power panel, and you'd probably be looking at powerlines based on g.hn (Devolo Magic type). If the garden office is of the "summer house" type, with mostly wooden construction, then you might get 250-300Mbps with just a directional antenna on the outside of the house (having first got the signal to the antenna) pointing at the office. To get any higher, you'll probably need a point to point link, as a consumer you'd probably be looking at devices from the likes of Ubiquity!
*There are TV antenna/Satalite installation firms in many areas that have branched into the field.
As for reliably getting Sky Q boxes to work with anything other than Sky broadband for some users, no matter what hardware they use, it seems to be a tall order!
50+ devices! If a great number of these are on the 2.4GHz band and there is a concentration of them in one area, then you might want to consider giving them their own access point (not just a different SSID/Guest network/or Mesh point).
Final recommendation: Get an expert who can perform a survey and whose feet you can hold to the fire if it doesn't work!
