According to the Thomson article the seating capacity is to be between 270 and 320 passengers with a 34" seat pitch in economy.
Looking at the Boeing specification for the B787-8, the seating capacity stated is between 210 and 250.
As the B787-8 is only a shade longer than the B763 I would think that Thomson may struggle to get 320 passengers in at 34" seat pitch unless they plan to increase the seating configuration across the aircraft to maybe 2-5-2.