What is a valuable or useful certificate or 'ticket' to have?

I posted this on UKjobs but was mostly ignored apart from one person. I'm willing to spend around £1000-£2000 on a training course or certificates but I have no idea which ones are likely to lead to steady employment. I've been unemployed for a significant period of time due to convictions. Most companies or organisations don't want to hire me, from supermarkets to councils, and from public transport companies to airports. When I see adverts online for construction, civils, plant operators, NEBOSH type work, they don't mention having to pass a background check, although I'm certain some will. I need something full time and permanent. A salary of around £30,000 before tax would be ideal for me as I live alone. I don't care about progression or gaining additional responsibility. I'd be happy to remain at the same level throughout. Although I mentioned construction, the issue with general labouring is, while it can attract a high hourly rate, it is often temporary, short-term work. Ideally I need something more long-term. I see some jobs requiring asbestos qualifications, NEBOSH, health & safety etc and most offer full time work but which one is best to have? I see the asbestos P402 is around £900, is that enough to get started in the asbestos industry, if there is indeed such a thing? Thanks for any advice from all you law-abiding, gainfully-employed citizens.