Philip, You look to be coming on nicely with the build, hope you've gone big enough... then it's 21' x 10.5' in £-s-d be it not.. should of taken up more of the garden.. less grass to mow..
... shorter walk to the house.. an less costly in getting the power out to it, the main of the cost to wire my shop will be the 10m Armoured cable.
Like the sound of the layout, ideal the toilet, if like me no sooner than I get out there.. than I have to trip back or I'm covered with shavings and have to go through all the house..
It is a good idea a room for the extraction system, I've found in my compact space, that the yorkclean I have can get shaving from turning it, which will, as with the bandsaw or belt sander blow dust round which is not picked up from them, so to duct it away somewhere else is ideal, where it can be done, the microclean is good, but at times for shop layout can fail to catch everything or then I need to plug it in as well.. which means I've to have something else to go off.. not enough points.. you'll need to plan for loads of sockets and lights.. I've a 4 gang and 2 gang cables and need to change out all the time..
what windows or sky lights are you having.. need to make the most of the North Kernow light, when you get it... one good thing with your mild St Egloskrug climate you may not need to add any more insulation, it's more a case of damp.
Finally when you come to having that BIGGER Lathe have you enough room..
... Shall watch as you get on with the shop going up.
David