Why would I request details of legislation and regulation from Pizza Hut when I can access (and have referenced) same directly from the Department for Business, Enterprise and Regulatory Reform website.
The entitlement to an unpaid break if you work 4 hours is zero irrespective of your age.
The minimum entitlement of over-18s if you work more than 6 hours is 20 mins no matter where within that working day you take your break (although it must not be at the start or end). It is not a sliding scale and no calculator is necessary.
But all of this is may be subject to agreement between employer and employees whereby compensatory rest may be offered to make up any and all shortfall.
If Pizza Hut allow for two breaks which total 20 mins or more so be it, but this is not a requirement that all companies must adhere to.
HTH.