I have great memories of the area, right back to the age of steam...... Some fantastic walks which get you away from the crowds.
As a matter of interest, my 'ahem' best time from the harbour clock in Torquay to home - 15 minutes off the M18 J1 is 3hrs45mins non stop in the Golf TDI, and one stop in the Bv8.
On our first trip overnight before we were married, and with no M42 was 4hrs 15 mins. Dolomite 1850 with overdrive and one stop.
I don't think those times are do-able nowadays. Worst time ever was over 8 hours during some serious flooding in October one year. A journey from hell, but the hire car was very good through deep water.
A friend with a 325 beemer once did it in 3hrs 15 mins in the early hours. It's roughly 275 miles from here.