We are looking to move to Florida Summer 2022.
We will have a 16 year old who will have just taken his GCSE’s and waiting for their results and we will have a 13 year old who will have completed year 8 at secondary school.
How will they both fit into the education system in Florida?
Education in the US is very localized. There isn't really an "education system in Florida." There are better and worse schools in Tampa.
I googled "top schools in Tampa." Looks like the top place is Sarasota County, Pine View School. The way it works is you move into the district that feeds into that high school. I don't know anything about that area, but if the school is that highly ranked, then probably it's more expensive to live in. But you can live in a smaller place in exchange for a top school.
So to cap it up: choose the district with the top ranked school. You can get this information via Google. Move into that district.
You can also do private schools, but they're expensive, e.g. Berkeley Prep is $30K/year.
In America High School is age 14 (9th grade) to 18 (12th grade). Your 16yr old will have to have the GCSE's converted into American credits, and if there are sufficient credits in all areas, be eligible to graduate early. If not, they will enroll in 11th grade where they will pick their classes along with any required classes to finish up the required number of credits. The 13yr old will have one year in middle-school, and then transfer to a different High School as a 9th grader. Given the cost of higher education in the USA, if college is in the plan, you may want to look at using local community colleges for the first 2yrs of school to knock out the 101 courses, then transfer to a 4yr school.
It was many years ago when I moved to the US as a 16yr old and it took more than two years to get school transcripts and visa issues sorted...a complete mess. In the end I'd finished another two years of school and had way more than the minimum number of credits required.
How the school processes non-American credit is extremely localized--honestly, it's almost down to the individual guidance counselor at an individual school.
If you run into any problems like @LilyWhites, reach out here. As a public school teacher, I can easily give you tips as to how to handle such difficulties so it doesn't take two years like it did for @LilyWhites!