Using extant VISA while appealing decision on new one

My partner has leave to remain in UK until August on a Tier 5 Youth Mobility Visa. She has gone to Canada to apply for a UK Ancestry Visa, having understood this could not be done from within the UK.

Her application failed because one of the documents she supplied was missing her grandfather's first name (which he didn't use, preferring his middle name). She is obviously appealing and currently procuring the extra documents that should make the appeal straightforward.

Once she has officially appealed from Canada can she return to UK to await the result, given that she is covered by her current visa until August? She has a ticket to Edinburgh booked for the end of February and no idea whether she can use it.

Really hope someone can give us some idea, we've been shuttled around every advice line and Home Office sub-department without any help at all. Thanks so much for any tips.