How important are GCSE's to good universities?

I joined a British school in London from an American school last September to do A-levels. The school recommended that I complete a Maths and English GCSE so that I can apply to UK universities. In my American school, we were doing pre-IB work while those my age did GCSE's here in the UK. However, I have seen on the course requirements for some universities that they want at least 3 GCSE's. There is simply no way I can do all the other GCSE's while doing A-Levels as well. Will only having two GCSE's affect my application? I want to apply to universities like LSE, Edinburgh and Bath

No you don't need GCSEs in your situation.

I was told by my school after explaining the situation that I needed to do an English and Maths GCSE since there were no standardized tests during my pre-IB studies.