Like several other respondents, I also did an ATCL (recital) recently (in March), and passed it. I also did (at about the same time) a DipLCM which I would recommend anyone to consider after Grade 8 (the London College of Music and Media is now part of Thames Valley University). I chose repertoire that was on both syllabuses, and had to play just one extra piece for the DipLCM. This exam is referred to by the LCM people as 'grade 9', and you only have to play for about 15 minutes - but it has a 75% pass mark, so it is easy for them to fail you. For this exam you also have to do sightreading, which counts for 20% of the marks, but no viva or programme notes.
I chose the Trinity exam because I sometimes accompany instrumentalists sitting the DipABRSM and Guildhall diplomas, and it would be my worst nightmare to enter for an ABRSM exam in my own right, fail it, and then find that I am accompanying someone in front of the same examiner - which could easily happen. I have never accompanied anyone doing Trinity grade or diploma exams, and so was pretty unknown there.
I chose repertoire wholly from the ATCL syllabus - which is apparently rare - most people get approval for own choice programmes. I only decided to enter at Christmas 2004 which was too late for this. However, it seems to me that the problem with own choice is that even if you have the approval letter, psychologically you will also somehow have the stress of justifying your choice to the examiner who might have some personal bias against your choice but can hardly cavil at the syllabus. The Trinity syllabus is perhaps a bit selective - and has a bizarre focus on some obscure Chabrier works (which are all in the Dover book). Also, it is a real challenge to produce non-superficial programme notes of only 300 words on 4 or 5 pieces. The DipABRSM allows for more expansive programme notes. However, in my experience the exam is often done by people in their late teens who do not often have the breadth of knowledge of their instrument or repertoire to do well in the viva, even though they can get external advice on the programme notes themselves.
I should say that I took the ATCL exam at Trinity in London and the facilities were superb, both the warm-up and recital pianos were excellent and the staff were friendly and helpful. The DipLCM I took at the Regis School of Music in Bognor where the piano was the same as used for grade exams and not really good enough for diploma performances in my view (they have a better instrument in the main hall which is used as a waiting room).
On the subject of degrees, I did a degree in Maths and Music almost 30 years ago. The music part involved some performance, but the degree itself was a BA, and as such is not immediately recognisable on a letterhead as having anything to do with music. I belately decided to do the diplomas so that I had equivalent qualifications to the people I accompany, to have recent experience of taking exams, and also to prove to myself that I can prepare performances to a good standard (I did not take any lessons or get any second opinions when working for the exams).
Phillip