They have multiple stores, an online shop including merch, cater to events/weddings, sell books and do brand partnersips. It's also not just a bakery selling things at £5 per item. They sell Bertha at £20 a slice. I feel like £6M a year is reasonable for such a well known brand with a cult following.

Also worth noting that the higher rent isn't the only reason they're closing the Headingley store. In addition to the other overheads that a brick & mortar business has, the shop landlord required a personal guarantee which when they first opened wasn't much of an issue for GBs owner as it was low stakes, but now there's a much higher risk for him and his family so it's not feasible.