You're quite right that the 11+ was a system that could be gamed, and people with lots of money would pay tutors to do it. One could make a similar argument about university admissions. However, it's also possible to pass the 11+ or get into Oxbridge or whatever academic "elite" measure you wish to use based on merit.

And it's not just about whether the school has more money. There have been plenty of government programmes over the years where schools could change their status in financially beneficial ways, and most if not all of them had nothing to do with being selective.

My concern is that academic ability tends to fit a bell curve. Most children will be somewhere near the middle, and will be able to progress at a similar pace. Towards either end of the bell curve, that stops working, and you need a more individually tailored approach to help each child maximise their potential. That's important to help children who have learning difficulties that can be at least partially overcome with one-on-one tuition, and it's just as important for children who are academically gifted but won't be able to take advantage of it or will even get bored and drop out if they aren't challenged at school. In recent years, there has been a lot of emphasis on special assistance for children with disadvantages, but it seems there's some sort of backlash against also trying to help children with advantages to make the most of them.

If you want a good way to stifle innovation within a society, holding back the smartest kids seems like a safe bet to me.

(In the interests of disclosure: I did go to grammar school and then study at Cambridge, and I did it without paying funny money to anyone. But I'm not really talking about me here, I'm thinking of a couple of friends who were far smarter than I ever was and who joined my school at 16. They had never really been challenged at their previous schools, perhaps because they were likely to get straight As anyway and there was no league table advantage in spending further resources on them. They caught up once they were working with teachers who were used to working with smart kids, but I've always wondered how many other kids didn't get in at either 11 or 16, never grew as much as they could have, never made it into the university they could have, and ultimately never received the education they deserved.)

Edit: Sorry, I forgot to address your other point:

And trade work isn't financially rewarding, that's a myth. The average wage of a plumber or electrician is 27k, bang on average wage. The only places they get 50K+ is in areas of shortage. I.e Rural Areas.

I think you're making my point for me there: in a generation when nearly half the population are coming out of education with a degree, a decent tradesman can still earn a comparable wage.

Also, since the trades are valuable practical skills, there is the chance to earn much more. There's a builder who's been buying up old places near where I live, knocking them down, rebuilding them with his mates, and then selling the new places for a very healthy price. The guy our landlord sends to do maintenance work from time to time runs a couple of businesses doing different things, and from what he's said if he bundled everything in together they might have to register for VAT. The threshold for compulsory registration is something like £77k at the moment, and while that's based on revenue before taxes etc. are paid on it, it still translates to a very respectable salary. When we had an electrician out a few months ago, the cost for him and his apprentice to so some basic work came out at a higher hourly rate than I've ever earned but was consistent with what others had quoted for the job.

Not everyone in the trades is going to get rich, to be sure. But there are plenty of people in the trades who are earning a relatively good wage based on their practical skills. As long as they're doing a good job, I have no problem with that at all, by the way. But why mess around pushing them to study academic subjects they might have little interest in, when they could instead work as an apprentice learning a trade they enjoyed and were good at? How is a degree in some dubious subject from a new university going to help someone like that more than a couple of years working jobs with a more experienced partner and then maybe getting some industrial qualifications so they can take on more demanding (and better paid) jobs in their field?