Learning Scales for Grade 8

I play a brass instrument too! (euphonium)
For minors, I just write out the sharps/flats that are in them on one side of a flashcard and the mane of the scale on the other then I put them in a wee food container box thingy( you could use any box or bowl) and pick one out at a time and test myself on what sharps/flats are in scale then turn over to see if I am right. Then I play that scale from memory 3 -5 times with those sharps or flats. This sounds weird but it really helps me. I find it best if I learn three one night and practise them then add another new one each night so I don't confuse myself. Also, make sure you learn the sharps/flats of the scale than just the sequence of fingering - I used to try to just learn the fingering but soon realised this was impossible for the higher grades. The more you practise the scale everyday, the easier it gets remembering the sharps/flats rather than having to work them out so you can play it more quickly and confidently in the exam
For arpeggios, just think of the scale of the arpeggio and work out the sharps of flats of that scale. Then, just play the triad (first note, third note and 5ht note) of that scale with these sharp or flats.
Whole tone - definitely harder but I think you only have to learn a couple (for ABRSM anyway) so I would really concentrate on learning these cos you know youll get one of the two of them whereas it is less likely you will get the one scale you dont know out of loads. Start on the starting note of the scale and play up the scale without any sharps of flats but when you get to a note where the interval between it and the next note would be a semitone, make the next note sharp or flat to turn it into a whole tone e.g for C play CDE (E to F is a semitone so need to play F sharp) then cos you had to sharpen the F, you need to play G sharp to make another whole tone, same for A sharp the you need to play C (because to be a semitone after A sharp it would need to be B sharp which is the same as C)
Tip - learn how to make the scale rather than the notes. Also, in exam, don't be afraid to take a few moments to work out what sharps and flats are in the scale rather than just randomly guessing quickly and getting it wrong.

Sorry for long post and hope this helps

Yeah I've been learning fingerings purely, and whilst I understand the scales (I've had to for G5 theory but also for Piano), I find it harder to play to a good speed. Does it matter if you play scales slow?

I have a book for ABRSM with the scales in and it gives recommended speeds to play scales at - for grade 8 I thought it was way too fast. When I did my grade 7 I played them really slowly and they didn't seem to mind. Think its definitely better to play them slowly and get them right than to play them fast but wrong. The speeds they give are only recommended speeds I think.