There are several ways you can do it the main two are:
1. Buy an established co, to get the loan you will have to put your house up as collateral, depending on what you pay it will take 5 or 10 years to pay off. I would not recommend this method unless you have some experience about employment law, h&s, running a business, organising staff efficiently (Not as easy as it sounds), bookkeeping (Its alright having a bookkeeper but if you dont understand the figures yourself they will suss you out in seconds and have your trousers down) You need to be familiar with all the standards its expensive when you dont install something to the correct standards and you have to put it right. You should also be able to sell, and when I say sell I mean make a profit not a fiver (This is where most go wrong they see their boss selling a system for £800 they think they can do the same job for £400 as the kit costs £300 they end up taking longer than they thought the job goes wrong and they have to go back and it ends up costing them money) remember it costs around £30 for you to go anywhere and do anything thats phone, van, fuel, insurance, labour sick pay, holiday pay, time whilst your van is off the road,
2. Become a subbie, the same rules above apply on a much smaller scale.
It can be done many people here have done it, many alarm co's are up for sale its a case of separating the wheat from the chaff its ok to buy a co thats not making any money so long as it has potential for making money, but as a beginner you need to be looking for one that is making money, they are much harder to find and much more expensive.
One of my ex-engineers is a subbie this week he is working on his house no work