Upvote:1
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.