As others have and will state, UA is in the process of updating the 777 fleet. Among the changes include: the new flat beds in business class, 3-3-3 seating in coach, and AVOD entertainment (although the Y options are not as good as F/C). Because so few 777s have been equipped and that UA doesn’t allocated specific metal to specific routes you have no way of knowing which 777 will be taking you to LHR (however 24-48 hours from departure the seat map/unitedcargo.com may tell you which version will be used with a higher accuracy rate).

While the Y cabin is going from 2-5-2 to 3-3-3 the new configuration will still feature some paired seats in the Economy cabin, specifically rows 20 and 31. 20 will be the bulkhead and 31 is the last row in E+ (near the restrooms and probably a little less desirable). I might try to grab the A/B or J/K seats in those rows and give it a try - that way you will be in the pair seats in either configuration.

I think going for the isle-isle in the center section and hoping for the middle to be vacant is risky this early given the limited number of newly configured 777s. The middle section in the new configuration is D-E-F in the old its C-D-E-F-G. A change between the configurations could result in some undesirable seats. D on the new (isle) becomes an inside seat in the old configuration.