You will need to follow the "route 1" conversion requirements listed here before applying to the airline:
Essentially it will involve sitting the ATPL theory exams (with a 650 hour theory course, unless you've already got ATPL theory credit on your license), some practical flight training before a skills test.
The validation pathway is not for you as that expires after one year.
Of course you should not begin to think about this process unless you already have the unrestricted right to work in the UK. That is a criteria on the BA website, suggesting they cannot sponsor you for a working visa.