Is she currently at a girls school?
Moving from mixed to co-ed would mark a bit of a change, but from single sex selective to single sex selective (assuming current school is selective) she shouldn't find any major differences.
The classes will be bigger of course. Partly because of this, she will need to be more of a selfstarter rather than wanting to be spoonfed -that is part of moving into any sixth form anyway. She might find the girls to be very clever (depends on where before) and find it a bit tricky at first to break in socially.
Tbh I think the issues will find are mostly those arising from joining a new school in the 6th Form, rather than the sector it is in. Apart from funding and numbers in classes, Tiffin will be very similar to lots of independent schools and of course, better in many ways. Many of the girls at Tiffin would have gone private if they hadn't got in there, although not all could have done - there will be a social mix but you aren't talking comprehensive style by any means.
For both you and your DD it will be important to NOT see this as a big issue - should be low down the list of conversation topics with other girls and parents, as should conversations about who has or hasn't got what - I'm sure you are very aware of this.....I Just say it, because in terms of fitting in, the only way it will be obvious to others that she has come from an independent school is if it is made a thing of.
I'm sure you'll find it interesting and there will be differences. Most of them will be due to a different school rather than a different sector though.....guess it won't always be clear which is which though and in your mind, it might all seem to be due to the fact it is State. I'm sure you'll find the gaps between Tiffin Girls and most independents in pretty small, if you can see beyond the shabbier facilities.