Why offer assistance in the first place, if you know you're in no position to actually deliver?

Because war isn't a RTS. Germany doesn't know what you do and don't have. You declare war to say, "Military assets are on the way, better back the fuck off." Now, maybe mobilization takes time, but either way, simply knowing Britain is now an actively hostile party is going to shape Germany's mentality.

Or, at least, it would. You have to remember that Blitzkreig was a very modern strategy, and Britain was operating under outdated doctrines and rules of warfare. So even if Germany didn't slow down (and they didn't, because that'd be entirely antithetical to their strategy), Britain is operating like it would.