Almost all security jobs i have come across reuire some level of security clearance, i understand that most this clearance comes from a previous job that have got security clearance, is it possible to fund clearance yourslef? cost?
im also unsure of how the security clearance process works, is it a worldwide thing or within national company? what things go against people for security clearance? im located within the UK and have previously been police cautioned a few years ago, would this stop me getting security cleared? and is there any time limit for this staying on your record?  what other factors come into this? is there any way of cleaning your record in the same way it is possible to clean your credit record? lots of questions here but cant find any documentation on this anywhere!

is it possible to fund clearance yourslef?
I think not. You must be sponsored by your job or the job that is hiring you.

cost?
Depends.. I think it can range from $5,000 + depending on the level of clearance.

im also unsure of how the security clearance process works, is it a worldwide thing or within national company?
As far as I know the clearance provided in the US is worldwide if you are going to be working for a US company.

what things go against people for security clearance?
Drugs, Theft, Credit history, Psychological background, Citizenship, etc...

im located within the UK and have previously been police cautioned a few years ago, would this stop me getting security cleared?
policed cautioned? Exactly what do you mean?

and is there any time limit for this staying on your record? 
For a Top Secret clearance, the background check will go back 10 years or from the time you were 16years old. Whatever comes first.

what other factors come into this?
Family members that are from different countries could be a bad thing.

is there any way of cleaning your record in the same way it is possible to clean your credit record?
As far as I know you can not.

Sorry if I am not very detailed 

This would definitely come up, but I am not sure how they would handle it.

From what I have gathered they are looking for links to foreign or questionable entities or have any of the characteristics that would allow you to be turned, see MICE (money, ideology, compromise, ego http://en.wikipedia.org/wiki/MICE_(espionage) )

All six (no exaggeration) of my close college buddies have gotten clearance of various levels so I have chatted with the background checkers a few times. I have also had more recent colleagues switch jobs to those that required clearance. All of my friends have gotten clearance so I don't know what will get you denied. None of my friends had a record of any kind.

One friend kept in contact with a foreign friend and they questioned him about that so be prepared to answer that truthfully if you have any penpals.

Not true. It only applies to certain government jobs and their contractors. There are lots of other security jobs in the civilian market with banks, financial institutions, retail... The list is quite long.

Yeah I currently work for the government and do not require a clearance.