Evo7 is absolutely correct when he points out that in the UK physicians, who are commonly known as "doctors" are using a courtesy title when they use "Dr" as a title. Courtesy titles should not be used in other contexts (but often are, particularly by members of the BMA!) Most of the public actually believe their GPs have doctorates! In fact the majority have bachelor or first degrees. (nb a first degree should not be confused with a first-class degree.)
After first degrees, the higher UK degrees are Masters degrees, and then Doctorates. For these latter graduates, "Dr" is a real title.
Another oddity of the UK system is that the title "Professor" is changed to Professor Emeritus, or dropped altogether on retirement.
Under UK law, titles are not protected and you can use any title you choose. However, what is illegal is to obtain any advantage by using a title you are not entitled to use.
For "Dr" to be a real title in the UK you must have been awarded a doctorate by a University that has a charter to award degrees. In the UK I believe these charters can only be awarded by the Privy Council, so the University of PPRUNE degrees won't count. I don't believe there is any similar formal definition for the title "Capt" unless anyone else knows otherwise.
On my first trial lesson, I was surprised to be addressed as "Captain" by my instructor. I later learnt that this was his trademark, and not all students are addressed in the same way by their instructors. Mind you, he called me a few other things as well when I was trying to get the hang of landing. Oddly enough, he was a physicist with a PhD too.