Will a btech in drama from the BRIT school be acceptable to go to RADA, or Guildhall or central, etc?

It may help you, but it will in no way guarantee you entry into any of those schools. They base their admissions on talent and promise; if you've been to the BRIT school you will have developed some skills to help you but if you're not more talented than the other thousands of people who apply, then you won't get in.