I’ve downloaded a route I’d like to do onto the 200. However I won’t be starting at the recognised start. Will the device pick up that I’m somewhere on the route and give directions from there or it will throw a wobbler not knowing where it is on the course?

It won’t throw a wobbler it’ll just shout at you and point you in the right direction. And then as you approach the route you’ll be able to see the line getting closer and closer until it beeps that ‘route found.’ You can edit gpx route you know and there’s plenty of sites that let you do it. Or just study the map so that you know the right way to go to get back to the route.