If you sub contract be prepared to get only 40% - 65% of the fee on average (less VAT). Some companies are good payers, other seem to take their own sweet time.
In all cases you are working for someone else and building up their reputation and market share. Few if any of the mainstream nationals will get on your back for attempting to be NDE so don't worry.
Good things: You get rapid exposure to a range of problems you wouldn't see in years. It's an income as you build your own business.
Bad things: Embarrasing to ask the customer for the rip off fees the nationals charge, even more frustrating when you realise how little you're being paid. You will need to build up a wide stock level to service the range of jobs you will be expected to attend.
Don't accept every job, think how much it's going to cost you to service a contract; ie. fuel, insurance, tools, stock etc. No good travelling 80 miles round trip for a lockout that earns you £26 less costs and tax. You might as well pay them.
As far as warrants are concerned, take it slowly. Make sure you are confident and equiped with all the necessary decoders and picks for the areas you work. Screw up and you will be binned
forever.
If you want to learn more about NDE and communicate with like minded souls in the UK you couldnt do better than joining the ICL at
http://theinstituteofcertifiedlocksmiths.org/forum/index.php?sid=a80e4944171584b59e3a92354050fbf1