I'm sorry, but it's not correct to say that schools are obliged to accommodate flexi-schooling by law. You are legally allowed, under the uk system, to arrange flexi-schooling for your child BUT no individual school is required to allow it in their school. It's entirely at the discretion of the governors and head-teacher and they can ask the LEA for their opinion, but they can say no. If they say no, you have to find a school which can accommodate it.
"The Legal Position
Flexi schooling is a legal option should the parent be able to arrange the agreement of
the head teacher of the school at which s/he is registered.
It is an offence for a parent to fail to ensure that a child of compulsory school age
attends regularly at the school at which s/he is registered. However the 1996 Education
Act states:
"The child shall not be taken to have failed to attend regularly at the school by reason of
his absence from the school (a) with leave" Section 444 (3)
In practice this means the Head teacher. To arrange flexi schooling therefore you should
prepare a proposal and set up a meeting with the head teacher. Whether or not it is
allowed is entirely up to the head teachers discretion."
The above quote is from the UK Home Education Org.
And it's also not correct to speculate that some of the girls at dd's group didn't go on to compete compulsory 4. As soon as a girl moves from the elite track to the grades they move to a different group, which never requires them to train in school hours. The gym is very firmly against skipping school for any other competitive route and I have to say that I can't see why is would be necessary. I work part time for BG and I'm not aware of any of the major clubs which require it for non-elite girls and I'm surprised if smaller clubs do.
All that said, I don't know if the OP's dd is on the elite track, she may be. If she is I'd say it's up to her if she wants to miss an afternoon, as we do. I'm only responding because I want to make sure they have the right information about their rights and other people's experiences when deciding what to do and it is something we have direct experience of and knowledge about, so hopefully can help.