My daughters more than made up for what they missed not being at school in the UK by the stimulation they got from living elsewhere, though they were at an International School. We came back from living overseas for Year 7 and the only problem has been that both my daughters can find their peers a bit narrow minded, obviously they learnt about other cultures but they also developed an empathy that applies to people from different social backgrounds as well. They have both ended up with their closest relationships being with other girls who have lived abroad or are of a different cultural background. Definitely a way of innoculating them from wanting to be one of the cool crowd!
And there are lots of workbooks that cover the Year 6 Curriculum to make sure she doesn't have to catch up when she gets back, but most secondary schools will do quite a bit of consolodation in Year 7. I found it very hard to get my daughter to focus on calculating the area of a football field when the sun was shining and we could hear the sounds of the adventure that was always waiting outside our door! You could ask the school she will most likely return to, if they are open minded they will be supportive because they will want the benefit of her experiences in classroom discussion. That was certainly the case with DDs school.
Putting children into local schools always seems to be very traumatic though. It takes a long time for children to adapt to the different norms and they can be miserable and feel excluded for quite a while, friends have had that experience even when the cultural differences are only slight eg French Schools (although given the values that govern the French education system perhaps "slight" is debatable!) and certainly when the culture is very different. In the case of where we lived there was the same emphasis on rote learning, which was a shock in itself, but also fierce competition. I suppose it depends on whether her peers in the new school will welcome her or see her as competition / different, you obviously know the school and it's ethos?
However you will need to check what the LA policy is on admissions. Our LA would only accept an application when we were back physically living at our address and could produce electricity bills in our name, even though we owned our house, could prove we were returning etc. By that time they were offering us a school miles away, god knows how many buses it would have taken! The state selectives wouldn't allow us to sit exams overseas either. Places did come up in the very oversubscribed local schools ( coed and single sex non selectives) either when the term started or within a matter of weeks but we lived close enough to go straight to the top of the waiting list. It is hard on a child to make them live with that level of uncertainty though and DD insisted on applying to very selective private schools not least because they went out of their way to tell her how much they would value her experiences overseas and facilitate her application, she sat the exams in her school overseas and they timed the interviews to suit us.