Couple of things:
1. I did the AC route from Bar Harbor south to my home in Philly back in '99. I wasn't thrilled with most of the riding in "Down East" Maine. I can only imagine that things are the same or worse development and traffic-wise.
2. Unless you are a low daily mileage person, two weeks is a long time to do Bar Harbor to Boston.
I disagree with indyfabz on #1 above. I think the Maine coast is amazing and well worth the trip. The scenery is beautiful and the small coastal fishing villages you go through are worth the trip on their own. Acadia National Park is worth several days; you could easily spend 3-5 days in that area alone and have plenty to do and see. I really don't think traffic will be too bad in mid-to-late May, except for Memorial Day weekend, as I mentioned. I wouldn't let that deter you. The Pacific Coast Route has plenty of traffic, but people still love riding that. Personally, I think Boston to Richmond would pale in comparison from a scenery standpoint.
For #2, I agree somewhat, but there are ways to fill that time and enjoy yourself if you aren't just about riding tons of miles every day. Here's a basic Google-map route of 384 miles from Bangor airport to Bar Harbor to Boston, via the Maine coast with some detours, Massachusetts coast, etc: https://goo.gl/maps/YuqGCaAp1Gs. Like I said, Acadia National Park and surrounding areas are worth several days. Maine towns like Camden, Boothbay Harbor, Freeport (touristy, but fun), the southern coast, etc., are all worthwhile diversions. And Portland, Maine, is a great small city, as is Portsmouth, NH. Down the NH seacoast is touristy, but pretty. And the northern Mass coast (Rockport, etc.) is nice as well. There's a reason that this whole area is popular: it's very scenic!
Or, for a longer route and more remote scenery at the start, fly into Bangor, Maine, and take a bus (http://www.flybangor.com/bus-service - scroll down to see "Wests Coastal Connection") from there over to Calais, Maine, on the Canadian border and ride back via the coast (a 478 mile route: https://goo.gl/maps/YwXeUAX3jBu). You'll see the "real" less-touristy Maine coast this way.
If it were me coming from an overseas flight, I'd fly into Bangor and transfer to Bar Harbor via shuttle (or stay over night in Bangor the first day, but there isn't much to see there), and stay there for a few days to get over jetlag, put your bike together (several bikes shops in Bar Harbor if help is needed), and explore the park and environs. Then, take the Coastal Connection bus shuttle from Ellsworth (right outside the park) up to the Calais area and ride back down the coast to Boston.
For flights, American Air (partner with British Air) connects to Bangor, Maine (code BGR), with regular service (small regional jets) from Philadelphia. AA and BA have several direct flights a day from LHR to PHL. From Bangor it's an easy ride (or shuttle) to the Bar Harbor/Acadia area. You can also fly right into Bar Harbor airport (BHB) from Boston, but it's on a tiny plane (3 seats across the width, last time I flew that segment). Fly into Philadelphia and maybe indyfabz and I can show you around for a day! :-)
Also, to clarify about Memorial Day weekend (May 27-29 in 2017) with regard to season traffic, it's a singluar event usually, with traffic and tourism spiking just for that weekend, especially up in Maine. The week/days before and after the Friday-Monday holiday weekend should be fairly quiet, with just typical shoulder-season tourism and traffic. In the northern Maine coast, the "official" season doesn't really start until almost July 4. Last summer we were in Acadia in mid to late June and it was pretty quiet.