Simply put, there's no 'one guaranteed way', all of the methods that you've mentioned are possible, and it's certainly going to help if you know someone there already or can be introduced by someone with a good relationship.
There's monster players out there with great visibility that aren't endorsed, there's awful players who shouldn't be let within 10 feet of a guitar that have their own sigs. The music industry (especially in the UK) never follows a set structure of 1 + 1 = 2.
I think another question to ask is 'Why do I want an endorsement?'. From the companies side, obviously it's about visibillity and potential buying influence on fanbase, but from the artist's side, other than vanity, what is the reason for it? You're unlikely to get any more exposure from the company unless your current musical project is already getting great exposure, and it's very likely that you if you do get a deal, you will be offered discount mainly on lower range instruments (as it's unlikely you've got fans who will spend £1000's to be like their heroes, in the same way that people will spend big bucks to get Petrucci's guitar or Meshuggah's guitar). Personally, if you're going to chase deals, better to spend the effort on things that will improve your situation (ie Amp deals with tour support for when you can't take your rig with you, string deals so you don't have to shell £100's on strings every tour)
Ultimately, I would say time and effort is better spent on improving your musical output and enhancing your reputation, which will eventually lead to companies being interested and coming to you.
Sorry dude, probably not what you want to hear, but I would say that's the reality of it.