I'm in that area with a teenager at Sullivan, and friends in most of the local schools, so some insight. Glenlola takes a broader range of abilities than some of the other grammars, friends say that pastoral care and support is good, teaching can be variable.
Sullivan (mixed) and Strathearn (all girls) are the non-Catholic schools with the highest 11+ requirements I think, although my kid finds the teaching a bit hit and miss at Sullivan too - it's like they assume all the kids are clever so they don't have to put much effort in, if you see what I mean? Friends whose kids are in Strathearn are very positive about it. English kids will be nothing out of the ordinary in any of those schools, and are unlikely to have any bother.
From Maxwell you could easily access dedicated bus services (not free) to the likes of Strathearn on the nearby Bryansburn Road, but it will definitely take a lot more than 18 minutes!! The distances may look small but the roads and traffic congestion in NI are awful. Just announced today that trains will start running from the new Grand Central station from 13 October, but I don't know if that means all services.
I presume the local primary PP referred to upthread is Grange Park. It has a good reputation but I personally can't recommend it, my older one was fine there, but the younger one had a rough time and they were less than helpful. Their safeguarding left a lot to be desired. They've had a new head since then though, so it may be better now