Hi Clare_C
I wouldn't under-estimate the 'cello! Like you I've done grade 8 piano years ago and am fine on theory etc. I took up the 'cello late last year, and I wouldn't be able to say easily in words how challenging this instrument is, especially for latecomers like us who haven't played bowed-strings before.
Zoda (hi again!) - I think grade 8 from scratch in 4 years would be very exceptional!
Liz's '8-10 years' would be more realistic I think.
It's one thing to just bow the notes which are written down - which the lower grades allow. But having read the expectations from the ABRSM on the higher grades, they expect great technique, little or no scratching or the usual 'noise' which can be eliminated but only with technique and practice and patience and good teaching - in my humble view!
I haven't taken any grades in 'cello yet. I've managed to 'get through' a grade 6 piece (Bach Minuet I&II, suite no1) without stopping, but I wouldn't want anyone to actaully hear it! Making that a grade 6 standard will take a long time in my view and will be best achieved by working up to it.
There is such subtlety, things which you don't see but only pick up with good teaching and perservence... (saw on a posting where someone is going to learn 'cello without a teacher??!! Not possible). Not to mention 3 clefs (ok you've got 2 of them already!). If you were allowed to pluck instead of bow, I'd say 4 years to gr8! Bowing has to be one of lifes great challenges!
Have you any particular reason for wanting to attain Gr8 as quickly as possible?
You will need a very good teacher and be prepared to practice a GREAT deal to achieve this quickly (agree with Liz very much on that).
I could be wrong though! There are always exceptions to everything....
I've just bought Rostropovich's DVD on Bach's 'cello suites, and he makes it look so easy its not true. But then he is one of the all-time greats (he's not half bad on Piano either - it'd make you sick - all that talent in one individual!!)
He says he 'went through' the 6 Bach solo suites when he was only 15! Mind-boggling!
Keep us posted on how you get on!