I should have added the following to my previous comment.
I just completed Morgan's Over America VII in my +8 which was 7600 miles in 49 days. We averaged around 200 - 210 miles on driving days with coffee, lunch and sightseeing stops, leaving around 8.30am and arriving at hotels around 4.30pm. The drive was 85% on back roads only going on interstates when absolutely necessary, the longest drive was close to 300 miles and that was enough by the end of the day. This was in the +8 so the M3W would be even more tiring and British back roads are more difficult in themselves.
Here is a link to some pictures:
https://www.tm-img.com/album/k2yH
