At what ABRSM grade would it be sensible to attempt the piano component of this sublime piece of music?
Well, it's always sensible to attempt a piano piece. It's not like attempting to skydive without sufficient training to make it successfully to the ground, after all.
That said, after looking at the repertoire requirements for the top ABRSM grade, grade 8, you would have to be well beyond this stuff to be able to play the Emperor with any degree of success. For example, you should be able to pretty much sight-read most Chopin waltzes, and work up Beethoven Op. 10 1 or 2 in a week without much difficulty, say a week of practice.
If you have a copy of all the Beethoven Sonatas already, have a look at Op. 81a, the "Les Adieux" sonata. It has similar technical challenges to the Emperor although it is much shorter. (It so happens it's one of his greatest sonatas as well.) If you can play that in a manner that you find satisfactory, you are probably ready to tackle the Emperor.
The score for Beethoven's Piano Concerto No. 5 at PianoStreet.com indicates that the level is "8+"
It seems reasonable to assume that their "levels" correspond to the major music examination bodies' "grades".
It is inadvisable to lump together pieces of music with other pieces merely because they are on an exam syllabus together. Music isn't grades. Some music is used to test one's progress in learning fingerings in small exams called grades. The piano solo part to Beethoven's piano concerto no 5 is not defined by its grade standard and cannot be played by someone who has only passed grade 8, and hasn't got any further in their learning.
What they might be able to do is to have a go at playing the notes for the piano concerto. If this is the case then the grade 8 connection doesn't make any difference to whether they can do that or not. If anyone wants to have a go at playing anything, the answer is to try it and see how they get on.
To play Beethoven's piano concerto no 5 so that the interpretation is as near as possible to what Beethoven intended needs a highly skilled professional pianist.
There appears to be a lack of education in the UK about the limitations of grade exams. For this I blame instrumental teachers in not making clear to their pupils what these exams test and also what standard they are in relation to general music playing.
I am sure that if the person who asked this question had had this information properly explained by their teacher so that they understood that grade 8 is an elementary exam and is part of learning the basics of playing the piano they would have realised that the best that they can do with the piano concerto is to have a go at the notes. If this is the case there are lots of pieces that people can play the notes in without needing to worry about whether they need to have passed a certain grade exam first.
If you like the piece and want to play it, then do it!
What is this grade nonsense anyway, but somebody's (maybe many people's) opinion that this is "harder" than that.
You can make rational judgments like that for yourself. If you find the piece is "too hard", you may at least gain insight into what kinds of techniques you need to work-up to get there. This may guide your practice toward the goal faster than waiting until you're ready.
There are many issues with the above post. While I stand by the content of what I wrote (as far as it went), the tone was unnecessarily aggressive. And it's oversimplified; it should have lots of caveats and appologies.
So instead of all that, I'll just try again.
Grading systems wouldn't exist if they weren't found to be useful by many people. But there's an exception to evey rule, and it might as well be you (assuming you wish to be "exceptional").
There is value in challenging oneself. And desire is a very potent energy source that can be used to feed the necessary practice.
But this piece may indeed be too much for your current skill set, and "pushing it too hard" runs the risk of things like uncontrolled muscle tension, which can lead to pain and physical damage. But awareness of this danger should be sufficient to mitigate it. If something "feels wrong" when you play, it probably is.
Even so, if you really like the piece, then go ahead and "attempt" it. You may be able to execute the simpler passages well enough to get enjoyment from it. And difficult parts can be taken at a very slow tempo (where you can concentrate on making smooth motions), or you can simplify them to facilitate playing longer passages. Knowing more about the piece and what techniques you will need can give you a head start on the right exercises and intermediate pieces to bridge the gap.