I wonder how much Greggs make on their 'own brand' juice and crisps. The immediate model that springs to mind is the starbucks one - where 'products' are sold from country to country until they end up in a country with no tax to pay.
EG Greggs could produce their crisps and juice overseas, where tax is much lower, then sell to Greggs UK at an inflated price. So the profit is made overseas and the UK Greggs just about makes a small amount to pay dividends to any shareholders.

It's for the same reason that most planes are registered on the Isle of man , why tech companies have the HQ in Dublin and why U2 moved their company HQ away from Dublin to Amsterdam - so they can pay less tax - according to what they produce. It used to be zero tax from Artists and musicians in Ireland, when that changed - Bono and Co jumped ship....