being a single parent, albeit male, with a similar question coming up, i think the only ones qualified to answer this is a Rabbi (or similar) that knows you, your daughter and the school well.
having said that, i grew up in Gateshead, (for better or for worse), i know Rabbi Katz, and i know the system somewhat, my sister learnt there, and i will say the following, that there is no question girls that go to Gateshead come out different people, the grow, they mature, they develope, they advance in every which way….
so yes, if you can manage the seperation, the financial burden, the downside of sending your daughter away then there is no question your daughter can have tremendous benefit from these two years.
in addition, i agree with tzippi, she doesnt need to come home 4 times a year, many friends will host her some of the vacations.
edited
good luck with whatever you choose to do and may G-d help you make the right decision.