The answer to your question is that you need to draw up a business plan and run it past a solicitor specialising in commercial law.
BG are authorised and regulated by the Financial Services Authority, and so anyone unhappy with a complaint made about Homecare to BG can have their complaint looked at by the Financial Services Ombudsman.
If you draw a up a plan of how you see your business working, and how you see it being financed, then a commercial lawyer will be able to advise you on what you are letting yourself in for. A lot of solicitors claim to specialise in everything, don't bother with one of those, you need someone who is a legal expert in this area. So do a bit of research before getting involved. The advice is going to cost you a fair wedge, so make your plans as clear and concise as you can - the longer the paperwork takes to read, the more it will cost you.
You sound to be frustrated by any mention of complexity, but taking advice early on may save you a lot of money and grief later on.
From what you have posted above, it sounds as if you are hoping to do this on a shoe-string, but no matter how deft you might be at your trade, you are going to need some capital to prime the pump with.
Rather than offering something that sounds like an insurance based scheme, maybe you could offer a boiler/system 'check and service' that includes an agreement to attend to any later problems based at a fixed charged rate.
So to start the ball rolling, let's say you offer a boiler service and a system check for £100 (the amounts are just examples). You service the boiler, and you check the system for faults, part of the ingoing deal being that you will fix any faults found on the system at cost price (whatever that is). Once you are happy with the state of the system, you offer to cover any further repairs for a fixed fee of £50 up to a maximum of £500 - but no boiler replacements.
If the customer is not happy to pay for any work required to bring the system up to standard, then you just charge them for a boiler service.
BG get people on HomeCare contracts by offering introductory fees, and then they jack up the monthly charge year by year after the first year. They claim to "service the boiler", but only do a gas check and a scant visual check. Their contract has all sorts of get-outs that makes a farce of their slogan: "looking after your world".
Repairs can take days because they are frequently: "experiencing exceptional demand for their service", and they weren't even answering the "24 hour emergency helpline" for extended periods during April and May of this year.
Hardly surprising that they get so many complaints when their customers become disillusioned by the reality of what they actually provide, having made all sorts of reassuring promises to get people to pay their monthly installments.
Presumably, you like to sleep at night, and so you won't want to sale close to the wind by making glowing promises that you don't deliver on?
The cheapest boiler and system cover I've seen is by Help-Link, who say they provide a boiler service and full cover of the system for £15.99 per month - approx £192 a year.
For anyone starting out, I would think £15.99 would be the benchmark to work to.
To have any hope of getting this kind of scheme off the ground, you will need other engineers you can call on. Whether you go the "insurance" route, or the fixed price scheme, if you let people down, your name will stink in no time at all.
If I were in your shoes, I would put some ideas down on paper and have a conversation with a suitably experienced commercial solicitor, and find out whether there is a business model that you can afford to follow. It may be that such advice can guide you towards a feasible business plan.
