Presidential candidates.?

Like in the UK, there are very few laws in the U.S.  governing the selection of the leader of a party.  Instead, each party gets to write their own rules with only some limits (and those limits are part of the problem).  

What keeps someone like Nigel Farage or Donald Trump or Boris Johnson from rising to the top is a system that: 1) normally results in the major parties showing good sense in picking a leader; and 2) allows voters to reject a party if they pick a person who should not rise to high office.  Of course, sometimes the system works and sometimes it does not.

The big problem is that potential cures are as bad as the disease.  For example, up until recently, the selection of a party leader in the UK was left to the party caucus in parliament.  The U.S. had a similar system for picking presidential candidates for about 40 years or so.  In both countries, there was pressure for a more democratic system that would give ordinary party members a voice.  And, unfortunately, ordinary members are more likely to be swayed by populist appeals.  (In the U.S., this tendency is made worse by the fact that, even in the states with the most restrictive laws, you can become a party "member" simply by signing up with your local election authority several months before the election which makes a hostile takeover easy.)

In the U.S. for the past 50 years, we have been tinkering with the process -- should we use a winner-take-all system in each state or should we use a proportional system; which states should go first; should party "insiders" have a certain percentage of delegates to the national convention and should they be free to vote as they wish (regardless of the results in their states); and should delegates be free to defect from their pledged candidates.  The rules that got us Donald Trump were, in part, an overreaction to Ron Paul's attempts to have his supporters picked as "Romney" delegates in 2012 and, in part, Republican rules that allow each state party the ability to decide the system for allocating delegates (which leads to some states being winner-take-all, others being winner-take-most, and others being proportional).  

Not to worry, the Democrats are working hard to ensure the US becomes a one-party state.

" -- do your laws or processes need changing to ensure the right calibre of person gets the Office?---".

That's even more scary than Trump, it's the "authoritarian politics" of both the left and right --- only those that "we" approve can stand .