As others have said, you are likely better off avoiding an area with grammar schools.
To give you an idea of schools in Surrey there is a government website that provides details for all schools in England:
https://www.compare-school-performance.service.gov.uk/find-a-school-in-england
So, for example, if you were to put in Epsom then it comes up with 9 state secondary schools, 5 independent schools and one state special school (for those not able to access mainstream education) within a three mile radius. (That is roughly Sutton, Chessington, Leatherhead, Banstead).
The site contains lots of information about each school population, for example, number of entries in each subject at GCSE (year 11) and A Level, number and performance of disadvantaged pupils, pupils with English as an additional language, split between girls and boys, how many pupils joined the school after the start of year 10 etc.
Then things like how many pupils there are that have SEN support or a Health & Care Plan. Total number of teachers and Teaching Assistants.
But I would say that the most important figures to look at are the Attainment 8 score and the Progress 8 score. Especially when these figures are split out by prior attainment.
Attainment 8 is a measure of pupils average academic performance across eight subjects. So, the higher the score for a school the better results the pupils are achieving. The average figure was 46.3
Progress 8 is a measure of pupil progress between the ages of 11 and 16, a sort of "value added" measure of how much the pupils at that school have progressed while they've been there.
If a school has a score of zero then its pupils have made average progress. If the score is above zero then the pupils have made more progress than average.
Around half of schools score between -0.3 and +0.3. The highest is +2.55 and the lowest is -0.8 for an entire school.
These scores are then further broken down by prior attainment. So it looks separately at those pupils with high, medium and low prior attainment at their previous primary school.
.
Just looking at the headline figure can give you a good start. For example, looking at the nine schools in and around Epsom it turns out that two of the three best schools are girls schools (Rosebury in Epsom and Nonsuch in Sutton) so that leaves them out for your DSs.
Up with them though is St Andrews Catholic School in Leatherhead with an Attainment 8 Score of 64.3 and a Progress 8 Score of +1.2.
If you break those scores down you see that the Progress 8 score is good for all three types of prior attainers (what this means is that regardless of whether the pupil was the brightest or not, all of them progressed well). The only are where they fall down is in science subjects for low prior attainers.
There are not many disadvantaged pupils at the school, 6% compared with 27% nationally and fewer pupils with SEN support (7% compared to 13% nationally).
The Glyn School is a boys school in Epsom and they also have good scores as well but not quite up to the standard of the three mentioned above.
.
If you contrast this with some of the schools lower down on the list, for example The Beacon School in Banstead or Epsom & Ewell High School in Epsom.
The Beacon School has an Attainment 8 score of 46.4 and a Progress 8 score of -0.14. Epsom & Ewell High School has Attainment 8 score of 41.6 and Progress 8 score of -0.39.
Taking the Epsom & Ewell High School it did below average on Progress 8 overall but it was particularly bad with high prior attainers with a score -0.68
That really is quite bad indeed.
For example, you would typically expect around 85-90% of high prior attainers to get GCSE maths and English at grade 5+. In this school only 65% of high prior attainers managed this.(along with 32% of middle and 2% of low prior attainers).
Disadvantaged pupils did particularly poorly with a Progress 8 score of -0.88. However, it was the 24% of pupils with English as an additional language (EAL) that did well in the school and actually outperformed compared to others.
Attainment 8 score for EAL pupils was 47.7, for others it was 40.1. For EAL pupils 46% got English & maths at grade 5+. For others it was just 29%.
.
It was also interesting looking at how many pupils transferred in after the start of year 10. At quite a few schools around 10 or 15 pupils had transferred in during this period. But at the best performing schools it was one pupil at most. People are not leaving these high performing schools in year 10 or 11 unless they have to.