What happens if I don't actually hold the head lease, just a sublease, when I try to sell?

Selling a business operating under a sublease adds a layer of complexity, because you're now dealing with two separate lease relationships instead of one. Assigning your interest to a buyer means assigning the sublease, and that generally requires the consent of your own sublandlord under the terms of the sublease — but depending on how the head lease and sublease are structured, the head landlord's consent (or at least their non-objection) may also be required, since a sublease is itself subject to whatever the head lease permits the sublandlord to do.

Your buyer's rights are also inherently limited by both documents: they can never end up with stronger rights than your sublandlord actually has under the head lease, and if the head lease is nearing expiry or contains its own restrictions, that flows down and limits what your buyer is really acquiring, regardless of what your sublease alone promises.

Because two sets of lease terms need to be reviewed together, not just your own sublease, this needs careful diligence before you price or market the sale. A Treadstone business lawyer can review both documents and identify where consent is actually required.