Warbler said:
hmmm, interesting. So technically speaking, the Queen could choose whomever she wants as PM.
If an odious far-right Candidate like the BNP leader Nick Griffin got elected as PM (Although this is more or less impossible for the reasons below) then the Queen could indeed refuse to except him as PM and it would be down to who the armed forces sided with to decide if there would be another election. I suspect as her Majesty's armed forces all swear allegiance to her they would back her up. There was talk of a secret millitary coup in the 60s/70s (I forget which PM) that almost happened because they thought the Labour party was in league with Russia.
Warbler said:
That still doesn't explain how Cameron got elected to Parliament in the first place. He had to get elected to Parliament before he could become the head of his party, yes?
The local party decides on who their candidate will be. A number of factors impact on this: electability, charm, influence, being the husband of the local party chairman's daughter etc etc.
If an area is a "safe seat" (e.g. the area has voted for a candidate from the same party for the last 20 years) then it's effectively the local party officials who select the MP. Of course if they choose to put up Hitler as their candidate they'd lose that seat, so they are constrained to choose somebody that appeals (To some small degree at least) beyond their own party grassroots.
So Cameron coming from a background of wealth and privilege could easily get elected as the candidate by his local party. Then if his seat is "Safe" he's in parliament... boom. It's not a perfect system but it could be much worse. At least this way a local born man/woman with little or no financial backing has the chance of being PM (Just by being well-respected/popular in their local town/city). However the evidence of the current Eton political set suggests otherwise :-(