Count on the 752, given that they've filed an actual 752 ship number and seat map.

I bet the flight is substantially oversold; be sure you have an e-mail address and phone number associated with your reservation, and expect a call or e-mail from the United "special offers" advance group who handles oversales 24+ hrs in advance of the flight. (You can usually negotiate an alternate flight in a premium cabin plus a travel cert in exchange for giving up your seat on a massively oversold flight.) Show up to the airport super early and you can swing the same deal, probably leeway to ask for $500+ in travel certs plus alternate travel on UA or OAL in J.

If you must fly as booked, check in early (reduce your odds of being involuntarily denied boarding), and it's probably best to request seating in row 20, which is completely open right now. The day flight EWR-LHR isn't too bad in a 752.