I've just been approved for a UK ancestry visa from Australia. But I've just realised the start date is three days after I'd like to arrive in the UK. Can I enter early, enter as a tourist and then re-enter later on the ancestry visa?
If you're eligible for a tourist visa, yes. However, you'll want to be very clear and upfront, especially if your ancestral visa is in your passport, as attempting to enter earlier than the date specified may cause your visa to be cancelled.
I'd recommend having your channel-hopping tickets printed out, evidence of accommodation and make it very clear that you will definitely be leaving before your ancestral visa is due to start, and returning then.