In a normal circumstance I wouldn’t build a minor on Scotland unless it was late in the game and the UK needed more production to fill transports heading to Europe. With that said as the UK you would be hard pressed to build more then 10 units per turn though because 5 inf/5 art is 35 IPCs. if for some reason you have more then that I would just sprinkle in some tanks. An early IC build on Scotland would defiantly get the attention of the Germans as it is often a stepping stone for the invasion of London anyway. Even if the Axis weren’t thinking Sea Lion, it would be hard to pass up a gift like that as Germany LOL.
Now in the situation at hand (after the liberation of London) it really depends on how much income/territories the UK has at the time, and where the allies (UK) want/need to build. Maybe the UK has lost most of Africa, and will only be generating 15-20 IPc’s and wouldn’t be able to build more then a half dozen units for the next couple turns. They could also conclude that their focus be elsewhere once London is liberated. Maybe the English have IC’s in the Egypt/Middle East and want to produce in that theater. If that’s the case then they wouldn’t need 10 production up there because with their limited income they simply wouldn’t be building many units in their Island kingdom anyway.
A miner added to Scotland would give them 6 production, and save them 8 IPCs (vs upgrading the London IC back to a major). W/Scotland IC added you can still build 6 ground units per turn, and transport them to mainland Europe. Plus with your transport fleet based in sz110 you would still get the benefits of the London AB/NB.
On the other hand, if the allies are planning on using London as a base for invading Europe, then a major would be the way to go (although it could be several turns before the UK could meet that goal of 10 units per turn).