Every gym is different. What might fit best at one age might not be best at another. I know of one situation where several high level gymnasts left a gym last Spring and moved to another. At least one of those was a girl in her junior (penultimate) year of high school. At least one of those girls later received a scholarship to a very good school.
Our girl, not one of those mentioned above, changed gyms and we are all happy she did.
There would be disruption though; having to make new teammate friends and losing some old ones. Maybe the drive is longer. Dealing with change in the last couple years of high school. If she is elite (that word has connotations in the US, I don’t know about the UK) she might go from being a big fish in a little pond to a big fish in a pond with lots of big fish!
I’d be surprised if a new gym wouldn’t welcome a new gymnast especially if she is a strong competitor. After all, you are a paying customer and gyms want solid teams.
Check out other gyms before you talk to your current gym, if you do. And do what’s best for your daughter and the family.
Good luck!