Also, Terri and I have disagreed before about the quality of schools. I have three very bright kids(in my opinion) and they've done really well here. We have never had trouble with the schools not being academic enough.
Everyone is going to have different experiences and opinions.
Look at the OFTED page. http://www.ofsted.gov.uk/ It inspects all the schools and will give you an idea of how the schools are in the area that you're looking for.
My kids are VERY academic and the school system here in our Council has really suited them well. Also, my DH is an academic and what he is seeing them do is waaaaay beyond what he did in high school and well into university. This is certainly better than what they were getting in the US. Honestly, my kids thought they had died and gone to heaven when they started school here and their opinions haven't wained (we have 5 school systems in three countries under our belts, so I trust their opinions!)
I think it does depend on the school system. the schools my kids are at, they will push them as far as they want to go and it's not based on labeling like what tends to happen in the US - it's results based.
Check on the University sites to see what the equivalencies are. The ones we looked at, they all accepted GCSEs and A-Levels and in Canada, they would get credit for A-Levels.
Also Leamington Spa is a small town and the school system caters to a higher socio-economic class so the schools might be what you're looking for.