You sit boards exam in class 10th in India, but in the UK "boards" or GCSEs are in year 11 based off the content from year 10 and year 11.
Admissions in the UK are age related.
If you are turning 15 between 1/09/22 and 31/08/23 then you will get admissions in year 10 in the UK and if you are turning 16 between 1/09/22 and 31/08/23 then you will get admissions in year 11 in the UK.
A friend of mine moved here to the UK from India and she was turning 16 that academic year so she got admissions in year 11. However, they only let her do 5 subjects (5 GCSEs) as it is relatively difficult to cover two years of content in one year. She later did a few more GCSEs after completing her A Levels (Year 12 and Year 13).
Most people do about 9+ GCSEs so how many GCSEs your school lets you do may also depend on how academically well you are performing currently. If you are doing good and can show evidence that you can cover two years of content in one year then they may allow you to do more subjects. Grading system for GCSEs is a 9-1 system where 9 is the highest grade you can achieve.
I moved to the UK from India in year 8 and I already knew so many topics that people were learning there because ICSE is quite ahead.
I'm hopeful for you because I realise Indian exam boards like ICSE and CBSC are a lot more content heavy and challenging than GCSEs are so you might already have learnt a lot of the topics for subjects like Science and Maths but obviously topics in humanities subjects differ a lot for different exam boards so that might be something you want to consider.
I hope this was helpful and good luck!