I am planning a multi-leg trip from BTV-Ewr-LHR-ARN and back.

I will be flying BA Traveler Plus from Ewr to LHR, but since they don't fly from BTV, I have to fly another airline with rigid luggage requirements. I'm sure I would have to pay a hefty extra bag fee. The same situation seems to be in effect for the LHR-ARN.

How do you handle such a situation?