Trinity-Guildhall offers several certificate examinations, i.e First Concert Certificate and Performer's Certificate, in violin subject (and I also believe in other subjects as well). However, if one gets any of these certificates, can one use it as a title at the end of the name? (Just like the title DipABRSM, LRSM, FRSM, LTCL, FTCL, AMusTCL, LMusTCL etc.)

The certificate usually tells you. All my dip certificates say...."Has the right to use the letters ALCM" etc... I'm not sure if a performers certificate is high enough up... I think you can only hve the letters if it is a professional diploma...

First Concert seems to be about Grade 6 ish and Performers about Grade 8 ish.

I've been having to refer to them to eliminate possible choices of piece for a singing ATCL, as with singing there seems to be a huge overlap - something Grade 7 one board might turn up as L on another or Grade 8 and F are sometimes interchangeable. Those are the extremes but certainly Grade 8 / Dip is quite common.