Well OP, there are dozens of excellent private schools in the UK and I don’t really understand why you’re looking so long and hard at WinColl - a school which from the sound of your post, you (and perhaps even your son) are not very confident with. And I suspect you’ll be paying full fees too to go there?
It’s a very expensive experiment, both financially and academically, if your experiment doesn’t work out.
Is it because your prep headmaster recommended it? Believe me, prep heads don’t always know what they’re talking about and I’ve got a living example of a premier UK prep school head making a complete blunder in recommending a pupil - his own son, no less! - to a certain senior school just to regret it woefully immediately after he had committed himself.
Having heard what you said about your son’s poor English and struggle with languages, I’m truly flabbergasted his prep head suggested WinColl of all schools. The UK’s top ancient private schools, both Eton and WinColl take their languages very seriously. As a matter of fact, these schools taught nothing but languages when they were first founded many centuries ago. Maths and the sciences were introduced much later. Don’t know about WinColl, many boys at Eton, for example, take four languages - two ancient, i.e. Greek and Latin; and two modern, i.e. French and English. Rightly or wrongly, they see these languages particularly the ancient ones, as a good stepping stone to the equally ancient universities.
And why would you want to send your son to a school where boys ask each other “how clever are you” and pretending to do no work themselves whilst in truth their parents apply immense pressure on them and tutoring them from outside the boundaries of what is to all intents and purposes a full boarding school - a concept designed fundamentally to prevent such pressures?