Welcome to this user forum.
What is implied by this post https://community.bt.com/t5/BT-Devices/Simple-questions-about-whole-home-WI-FI/m-p/1921343#M7905
Is that you can add an extra disc to what you have already, if you want a seamless system, if not, then use the cheaper option of a wireless access point, especially if the garage and workshop are out of wifi range, and would not form part of the mesh.
Also bear in mind, that six discs are the limit, so you would not be able to add any more.
If you have an area between the house and the outbuilding which has no wireless coverage, then a decent wireless access point, with the same SSID and password as the main network, would probably just as good, as devices are going to disconnect anyway, when you move between locations, so you will not have a seamless connection.
The TP Link TL-WA801ND costs about £23, and would be worth a try.
The extra disc would also work, but I think it cost a bit more.