The B787 EMA-CUN turned back with a Hydraulic failure.
All the questions about an aircraft "based" at a particular airport mean nothing as there are no planes "based" at that airport. Thomson has always had the capability in both planes and crews to substitute an alternative reg/type with another to keep the flying programme on schedule. Despite what looks like one reg/type at an airport for maybe a week or so, the aircraft are rotated through main bases for maintenance checks etc on a regular basis
I only agree that the 757 is one of the best planes for the job that Thomson do, but as others have pointed out, despite various airlines pleading with Boeing to make a direct 757 replacement, Boeing have no inclination/money to do so, so it will be a constant stretch of the 737, but remember.....they still have only 4 main brakes and so, as the 737 weight increases with more passengers etc, then the landing performance becomes even tighter because it will never have the landing performance of the 757 with 8 brakes. The 737MAX is the type chosen to replace the 757s. The A321 has other performance issues on the Thomson routes/loads as again, only 4 brakes, and a very small wing chord