I've taught for most of my career in mixed age classes, and I enjoy it and think there are more advantages than disadvantages.
I think the small schools that tend to have mixed age classes usually have a good sense of community, and all the children are well known as individuals. Also, I usually get to teach any particular child for 2 or more years, so I don't have to spend time at the start of the year on getting to know them and their abilities, and I build up strong relationships with families etc.
I agree that it works best when it happens every year and the school is really experienced at planning and teaching in this way.
I disagree that 3 form entry is best - I would have been horrified and overwhelmed as a child at being in a year group of 90, or a school of 700, at the age of 4. Small schools are somehow just more human-sized.
And while big schools have more money, that doesn't always translate to better access to resources. Would you rather your child had one scheduled hour a week in the ICT suite (as is likely in a big school), or spontaneous access to the laptops almost whenever the need arose (because there are only 2 or 3 classes to use them) so that they can be used in a realistic way for research or maths or phonics games. Plus a scheduled ICT lesson of course. And the same argument applies to such things as musical instruments, access to the hall, or field, or wildlife garden.
People sometimes send their children to private schools to get small class sizes - the numbers in the classes in my school are (I think!) 17, 23, 21 and 15.
If you want a C of E school, certainly don't be put off by it being small.