CBAP is a certification that is administered by IIBA (International Institute of Business Analysis). This is a globally recognized certification, which adds huge credibility to your profile. Whereas BCS or British Computer Society is a Society that offers Business Analysis certifications including Diploma. Now, BCS certifications hold a good reputation and add credibility in the UK. But comparing the two, CBAP from IIBA is more credible than BCS in global perspectives.
If you wish to get trained and certified in CBAP, will recommend you check out the CBAP certification course from Intellipaat. If you are just starting out, then look at the YouTube video on CBAP certification.