I agree 100% there's no need for entry and exit stamps. But unfortunately, many countries in the world, including the EU, still insist on them, so the delays will continue. When UK was in the EU, there were no stamps for UK or any other EU citizens, so things went faster and more smoothly.
As far as I know, John, with Kenya remaining in the Commonwealth, you can enter and exit "The Mother Country" with the passport Her Majesty issued you.
My Niece, born in the UK of an American Mother and Father, is a "dual". She now lives in Australia (born an expat, always an expat. I guess) and when she comes back here, she just lines up in a "returning US Citizen" queue. When she goes "home", a Commonwealth queue with her HM Passport.