To be perfectly honest, they were my initial sentiments but having just completed a JOGLE it's all rather irrelevant. What I mean by that is that I got off thetrain at Thurso and passed within 3 miles of Dunnet Head. I'd been there before by bike and didn't see the need to revisit it. Also as you get to Penzance you are probably within 12 miles of Lizard Point, so all in all you are talking an extra 30 miles on to an end to end trip. If you are that hung up about it then do it.
Having cycled 940 miles in 8 days, I had an amazing sense of achievment and adding another 30 miles would have proved nothing and meant little. I was at the stage where I wanted to get the job done ASAP and get back to my family. I loved every minute and wouldn't have changed what I did.
As others suggest, when you say you have done the End to End people are very impressed. Dunnet head and lizard point are likely to mean very little to anyone that you mention them to.
In summary, it's your trip, your memories, do it your way. There is no right or wrong.