If you're driving a goods vehicle over 3.5 tonnes the EU rules apply:
https://www.gov.uk/drivers-hours/eu-rules
Other goods vehicles are exempted from the EU rules but covered by ones for England, Scotland & Wales instead:
https://www.gov.uk/drivers-hours/gb-domestic-rules
(There are separate rules for Northern Ireland)
The EU rules are summarised neatly in this leaflet:
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/409716/staying-legal-HGV.pdf
So (under the EU rules) you can normally drive for up to maximum of 9 hours per day. You can EITHER drive for 4.5 hours, take a 45 minute break and then drive for a further 4.5 hours OR drive for 2 hours, take a 15 minute break, drive for 2 hours, take a 30 minute break and then drive for 4.5 hours. If you adopt the former pattern then, on no more than 2 days per week, you can drive for an additional hour after taking another 45 minute break.
The number of hours you drive in a single week can't exceed 56 but the number of hours driven across two consecutive weeks can't exceed 90. (So if you drive for 56 hours in one week you're limited to just 34 hours driving in the next).
Based on the 'single break' requirement in your question (and assuming that it's the EU rules which apply, and not the GB ones), you can drive from 0300 to 0730, take a 45 minute break and then drive from 0815 to 1245.
More here:
https://www.gov.uk/guidance/drivers-hours-goods-vehicles