True. It needn't be in the bathroom, but really you do need at least one radiator fitting.
If you are having a combi fitted then presumably you are thinking of having radiators installed at some point anyway? If not, then why not go for a multipoint water heater, cheapest at the moment is the Baxi.
You might have been better capping those tails or at least using a gate valve or similar to increase the resistance through. It sounds like the CH flow is simply going around the path of least hydraulic resistance ie. the tails, instead of through the new radiators.
The boiler will fire...