The biggest problems in my opinion would be the CD add-on, and the software library.
I don't see many people in Britain buying the add-on, definitely not in numbers, that sort of thing is unpopular around here, they would've had to have put more support behind Hu-cards.
The software would've had to have branched out into more areas, especially sports, however, it would've been in a very good position if released here ~1989, because Shmups and general arcade ports were very popular during that time, if it had become entrenched during that period then British software houses would've started supporting it by the early 90s (when scrolling shmups rapidly started to fall out of favour) solving that problem with little effort.
I've run into many British magazine articles about the PC-Engine now, they are universally positive, all the magazines like it more than the NES, Master System, and initially Mega Drive too, reader letters also often mention the system to ask when its coming out. I'm not sure if you've been reading the Euro info thread, but Phantar seems to think that opinion was positive for the PC-Engine in Germany too.