There is also Percy Hedley, which is somewhere near Newcastle; but having travelled thousands of miles myself with a child in residential specialist schools, unless its the only place there is for your child, then driving up to Newcastle and back from Surrey regularly to attend meetings like parents' evenings, annual reviews, school fairs, etc would be exhausting!
Even a drive, which should take 1.5 hours on motorways, can be 5 - 8 hours if the motorway is closed (like the M25)!
If you search on line, you will find other specialist residential schools, which name speech and language among their specialisms - but they won't be specialist speech and language schools. Say all the children have a combination of MLD/SLD/EBD/cerebral palsy/ASD then its likely they might have speech and language difficulties; but they are not going to be a suitable peer group for your son, who presumably is in the normal intelligence range (if he is in mainstream in Y6) with a developmental language disorder.
I suggest you look at the peer group for your son, when you look round any school - and ask to see the children in the class and residential accommodation he might go into.
Ring the Afasic Helpline - they will be up to date, if any new schools have opened up.