We've been through the same. Son was the only one from his primary going to our nearest grammar school (not as far as 10 miles but journey takes over an hour due to poor public transport). Son and ourselves were 100% sure it was the school for him, our only worry was the isolation and travel. We needn't have worried.
He's made friends on the bus from other forms and years (service bus as not even a school bus provided). He also found lots of others in his form were the only ones from their primary schools, (they spread them out and tried to avoid kids from the same school in the same form). So, they were all in the same boat, with few past friendships brought in, so could all start to form new friendships from day one.
I think grammars tend to cast their net far and wide for their intake, so won't really have "catchment" primaries where lots of kids go from. My son's has kids attending from all over the place - must be 30-40 different primary schools feeding into it.
Couldn't believe how happy and settled my son was. First day home, he was full of it, made a new friend straight away because they both played minecraft, so had something in common and bonded over that, and they were playing online together that same evening!
Looking back to my own school, I think I'd wished I'd gone somewhere else too. As it was I was in a catchment primary, so 28 out of 30 in our class went to our nearest comp. My best friend at primary was in my form, along with 5 or 6 others, so we walked there and back together, sat together, etc. At the time, it was good, but it meant I didn't really look to make any new friends as I was quite introverted and shy anyway, so stayed in my comfort zone. However, as years passed, and the form turned into sets/groups/options, I found myself very isolated once my primary friends were in different lessons/rooms etc., - New friendship groups had formed and I was an outsider.
As it is, it's been the making of my son. I'm now actually glad he didn't go to a school with his old primary friends. He's still friendly with most of them and they still go to parties together, go to eachothers houses, go to footie together, so he's actually got the best of both worlds - still in contact with local old friends but now has a new group of new friends via school.