If you are interested in ABRSM diplomas, I recommend having a look at the video they issued in 2004 (and which is available for sale) which shows candidates at various levels and on various instruments reprising parts of their diploma performances, together with interviews with examiners etc, and examiners' comments on the performances. As regards the FRSM, I was amused to see that for an example of the standard required, they showed the established concert pianist Vanessa Latarche playing a piece of Godowsky - as if to say, dont try this unless you are really serious! As a matter of fact, the fees for the highest diplomas are sufficiently high to put off time-wasters.
The key point with all diplomas is - however well you can play at home or even in public, can you cope with doing a 30 min+ recital in the sterile environment of an examination room in front of one or two examiners?
Phillip