I'm no expert here, but from a business point of view surely Thomson would want this plane back on the ground in the UK after hearing of the incident at LHR?
If the 787 were to be grounded again, i'm sure it would save them alot of hassle having the a/c at their base airport, for works to be carried out with minimal disruption? Not to mention the return pax being stranded over in the U.S, they would have to organise for them to be returned, at great cost/delay.
To me, seems a bit of a coincidence for this to be turned back at the same time.