It was possible once.....
http://alae.org/downloads/techlog/issue312.pdf
read the article starting on page 6....
"A number of BA employees in Europe held BA authorisations based upon A&P prior to 2001. This gave them an entitlement to grandfather rights. The A&P was also acceptable as a baseline qualification for an ‘A’ authorisation and would qualify as protected rights for a Category A licence.”
I think It was only possible (illegally) at the time of cutover to EASA Part 66 from BCAR section L. Now nobody would touch it with a bargepole seeing that article