I'd appreciate some serious advice. They won't know any local friends let alone girls if they have been boarding and disco's at girls school doesn't give them much conversational or other experience.
I'm afraid that in this situation you will have to accept that they will not get to know girls in the same way that boys at day or co-ed boarding schools can easily do. This is the downside of the few hugely successful boys' public schools which can safely stay single sex because they will never be undersubscribed. You have to decide whether the environment this provides for them outweighs the fact that they will almost certainly grow up regarding girls as a different species. Perhaps you should factor this into your final decision about what school to choose, as it will definitely colour their relationships with girls and women for many years to come, and maybe for ever. I have had boys at single sex and mixed schools, and the difference is very apparent to me, even though they have a sister.