They have in the past, IIRC. However, they may have found it's more profitable to use the 752s on the ps routes and the 764s to LHR than vice versa. Or perhaps TATL traffic was slightly less down this year than it has been in the past. I know there were a lot of complaints about 752s TATL because they tended to need to make a fuel stop in Canada on the westbound flight during the winter, but I think LHR-EWR is short enough that it shouldn't be an issue.

The one thing I'm sure of, though, is that running one large plane instead of two small planes -- OP's suggestion -- would be a big problem due to slot utilization rules.