Yeah, it could be strongly argued that the film might be better served if they'd rolled the ending credits after that last shot of the Boeing 767 flying off into the snowy night, followed by that panoramic vista of the Palace of Westminster and the Thames. It would've been hard to fault them if they'd not done the last 23:20 of film that followed that scene. The film would've been about 87 minutes long, but that's maybe five minutes shorter than the typical light comedy. They could've easily made that up with a little more padding of the London trip.
Then again, they had five days, (less one, when travel time is taken into account.) It's just enough time to hit the big tourist traps (assuming you start early, and crash late.) Given that this is the ADHD-Team (theme song: Dun-da-dun-duuun . . . oooh, shiny!) we're talking about here, it's somewhat difficult to imagine the London arc being made much longer without starting to feel cliche (haha wacky Japanese tourists and tourist traps) and repetitive (given the nature of the girls, it'd turn into Just Another Afterschool Tea Party . . .
In SPAAACE LONDON!) Especially since you'd have to re-architect the whole film to minimize the whole "we should do something for Azusa" plot; or re-work that subplot to move a lot faster (which, arguably, would hurt the opportunity to build on and expand the interpersonal dynamic in ways not already covered in the show. It's also, technically, doable . . . but Azusa knows better than to leave her "reliable" senpais on their own for any length of time.
)
Spoiler for Singing in the rain, I mean English:
I think the movie does a pretty good job at establishing that the girls' spoken English is, well, terrible. Learning to sing a whole song in English overnight, in a credible manner, would've been a bit of a stretch, even with the Hirasawa Instant-Learn Perk.
To be fair, Yui did try a little changing up the song as a shout-out to the venue. It's a time-honored rock tradition. It didn't earn her any more polite applause than they got for their straight performance of Fuwa Fuwa Time. (And, truth be told, they got a much better reception at the sushi bar.)
As a side note, all the English speakers in the film are native Brits. It says so right in the credits, as they all recorded their lines at the same London studio that Bohemian Rhapsody was recorded.