How do Trinity Guildhall and ABRSM compare for grades 5-8?
Do you have to do Grade 5 theory to do grade 6 practical in TG?
Are there any benefits of doing TG over ABRSM?
Your thoughts please ......

TG I think you need theory for grade 8 but not grade 6. You get a choice of supporting tests too.

Not quite correct: theory is not needed at all. The old Guildhall exams used to require theory for Grade 8, but when Trinity and Guildhall merged this requirement was dropped.

Thanks. Interested that theory exam not needed to do grade 6. Also I have heard testing is done by a violinist and violin related skills such as bowing are tested. Please confirm or otherwise.
Are the standards similar?

I think that it depends on the centre.
The centre that I generally use has one examining day a session and seems to have at least 3 examiners.
Among these I think there is usually a woodwind and a string examiner.

It might be LCM I'm thinking of. When my second and third children did grade 5 theory (together) they were both approaching grade 6 and teacher commented that one of them needed it but not yet and the other didn't need it at all. One was doing TG brass and the other LCM piano. (By need, he meant in the sense that it would prevent them from doing exams. He agreed with me that they should do it anyway)

Yes, the standards are the same.

Teachers tend to have a preference for one or the other, or choose which to use depending on the student. You can find examples of the same pieces being set at different grades by different boards, but that doesn't mean the exam is easier or more difficult as a result, just that they're looking for different things. To put it another way, something such as the Debussy violin sonata, on the ABRSM Grade 8 list, could just as easily be a diploma piece, or be used in a recital at music college.

All the information about exactly what's involved is available from the boards' websites, although it tend to be spread over several downloads, with aural tests for example being separate from instrumental syllabuses.

It is LCM who require grade 5 theory for grade 8 practical, Trinity has no theory requirements at any level although they do offer theory exams (and diplomas if you're a real theory nut, unlike ABRSM).

See these threads-

Are Trinity and Abrsm the Same?

ABRSM or Trinity?

ABRSM is much easier than Trinity?

(P.S Some are quite old, so things may have changed between then and now.)