Hi foxalfa,
I have absolutely no knowledge of this company whatsoever and have no axe to grind. My comments are purely objective.
You have to make your own mind up, obviously, but I would read through this thread very carefully from start to finish and then remember that (a) anyone can put together a very impressive looking website and (b) you can buy as many 'likes' as you want for your Facebook page.
I am not suggesting this is what the owners(s) of the company have done just saying that it is possible.
As for paying a "guarantee" to work as a FI, what on earth is that for? The aircraft must be insured, the students must be insured and as an employer they must insure you. The only thing I can think of is that this is the excess charge applied by the insurance company, in case of accident.
DO NOT under any circumstances take a job with anyone as a contractor - the chances are that you may not be insured by the company's insurance, regardless of whatever the company may claim. If you had any accident you could find yourself financially crippled for a very long time! At the very least make sure you read the insurance policy document to make sure you are covered.
Lastly, before accepting any job you MUST take a look at the company, the airfield and especially the aircraft.
There are companies hiring in the UK - you just need to look around and be prepared to work hard for little money.
Good luck - whatever you decide keep us posted.