One reason (among many) not to purchase, or at least think hard about the penalties of purchasing, a basic economy fare.

EWR-LHR is all (or at least, mostly) the high-J 767-300s. The pairs in economy are almost all E+. Except for a couple of pairs in the back, the only non-E+ seats are the middle section. You can not select those - though some BE buyers have gotten lucky and have those as their assigned seat at checkin - possible, but IMO, unlikely and definitely wouldn’t assume that would happen. Presumably, the pairs near the back go very quickly to folks who either purchase a standard fare and don’t get/want to pay for E+, or those who want a confirmed pair but don’t want to pony up for E+.

Again, be very careful about if you can get what you want from BE fares. Same reason I have never purchased tickets on ULCCs both here and in Europe - every time I’ve priced them out with the things I want/need - convenience of airports served, cost of transportation get to said airports, cost of a seat assignment and/or food/Bev combined with cost of the ticket (ie, the all in price) and the lower sticker price of the LCC were never cheaper, or were so close it never was worth it, especially considering the miserable onboard experience (28-in pinch in ‘pre-reclined’ seats - no thank you!). Buyer beware.