You are confusing 2 sets of regulations. The Tachograph hours regs say that after 4 1/2 hrs driving you must have a 45 minute break (or one break of at least 15mins during the driving time, plus one break of at least 30 mins at the end of the driving time). The The 6 hours you are on about comes from the Working Time Directive (WTD) regulations, which states after 6 hours of continuous work you must have a break. However these are as complicated as the tacho regs. WTD regs for transport workers are at
http://www.dft.gov.uk/pgr/freight/road/workingtime/rdtransportworkingtimeguidance
A little light reading! Bear in mind, that when driving the tacho regs take precedence over the WTD regs. Don't you just love red tape?!