|>>|| No. 23365
RM are still around, although they stopped making computers last year to focus on education-specific software and services. I don't recall them ever using a proprietary OS, although they did have a variety of "learning environment" software that presented a simplified interface. RM were Wintel from almost the beginning, as you can see from the magazine adverts posted by OP.
Funnily enough, you sort of hit the nail on the head. Along with Acorn, RM were well ahead of the curve in terms of networking. RM sold packages to schools which used a proprietary low-cost networking technology to share hard drives and printers; Using these shared resources substantially reduced the cost per computer. Acorn had similar technology (Econet), but they didn't provide the kind of complete service offered by RM. Schools were expected to go to local Acorn dealers for advice on procuring and installing a system, but the quality of that service was highly variable. In many ways, I think Acorn squandered the huge advantage they had gained through the BBC contract by failing to fully recognise the importance of supporting schools.
The first computer I ever owned was a cast-off RM 386, fished out of the skip at my school. If I remember rightly it wasn't perfectly IBM compatible, which necessitated various weird workarounds to run DOS games properly. The PE teacher at my secondary took a bit of a shine to me not like that, you rotten old cynics and tipped me off whenever the school was chucking stuff out, so I learned my trade by cobbling together computers from knackered old tat. I expect that there are still half a dozen old RM machines in my mum's loft, in various states of disrepair. Funny how little things like that can determine the course of your adult life.