Even if the 12 hadn’t gone to HEx, there wouldn’t nearly have been enough 387s - there’s nineteen 769s coming to GWR. With the 12 sets now on Heathrow duties it’s a total non-starter with the current fleet of 33 classic 387s. Perhaps if 101-129 and 301-306 were available for import from their current operators there would be enough to do the whole combined fleet of 80 (including HEx), put a little more resilience in the HEx fleet (10/12 units is tough on both operations and engineering departments, 10/14 like we had with 332s would be more comfortable), cater for wiring of Oxford/Banbury/Branches and see GWR-LTV move to an all-387 operation. But even then it would still rely on the technology being capable enough. The same struggles that the 769s are likely to have - IE generating/providing enough electrical power away from 3rd rail / OHLE to get the units over the hills of the North Downs - are going to be just as likely to occur with a battery 387 if the forays into battery tech so far are anything to go by.