1) Look on Unis websites for the courses you want to do and check the Entry Requirements - most will list BTEC requirements. If they dont, emai and ask. Be aware that many STEM subjects, especially at top Unis, will often want an additional A level.
2) Specified GCSE grades are not an optional extra - if a Uni says it wants 'GCSE Maths at grade 6' or whatever then that IS what they want and high grade BTECs or A levels will not make up for that. Applying without required GCSE grades is the quickest way to get a speedy rejection. If you are in Y12 now then ask your school about resitting any required GCSEs next June - any Uni offer would just include this as an extra condition.