IMHO, this also applies if you want a research career. At the end of the day, you eventually need a permanent position. Most professors at top UK universities got established in Academia by doing a PhD and getting a lectureship straight after their PhD or sometimes a postdoc. Lectureships demand significant research and teaching and supervision experience.
A high-end fellowship like the ones you mention is also fine if you want to improve your publication track record. In some fields getting a lectureship after your PhD or postdoc might be extremely difficult as the market is too crowded.
A high-end fellowship like the ones you mention is also fine if you want to improve your publication track record. In some fields getting a lectureship after your PhD or postdoc might be extremely difficult as the market is too crowded.