I'd be relaxed. If the price is more or less in the range what you'd have paid for a used bike of that age it sounds like a fair if not good deal to me. Regarding the wrongly lined out age: Mistakes happen and those kind of mistakes typically happens with Brompton dealers that do not sell loads of Bromptons and thus are not deeply into the model history. In between 2014 and 2016 there have been no relevant changes on the Brompton and yous is from Dec 2014, thus probably already a 2015 model year. Relevant changes have happened in 2013 (rims, cranks, brake levers etc.) - those are obvously on your bike. The next bigger set of changes (apart from the rear carrier that you do not have anyway) happened in 2017 with stems, bars and shifters - you would not have those with a 2016 model anyway. So in my eyes a 2014 bike is in no way worse or woth less than a 2016. The bottom bracket has been exchanged. The warranty starts with the sales date, no matter when the bike was build.
In fact the only thing that happened was that the seller made a mistake regarding the build date but in your case it does neither make a technical difference nor has it any effects. Full list of changes in 2015 and 2016 (apart from special editions, accessories and colors:
2015:
minimally different surface pattern on the saddle
slightly different material and surface pattern of the mud flap on the front whee.
height-limitation insert for the seatpost
2016:
new bell
new rear carrier
new roller wheels
So nothing relevant that you would miss. You do not have a rack and the roller wheels and height insert are easy and cheap to retrofit by yourself in case you want them. If your Brompton dealer is a really nice chap he might just throw them in for free as an excuse for the error and post these parts to you. I would be relaxed - if you liked the bike before so much that you bought it why would you suddenly not like it any more now just because it turned out to be two years older than you thought initially? The bike has not changed. And would you do the same with your girlfriend or wife?
