Before you start, you will need a method of transport and tools.
When you start, you will need public liability insurance. This, depending n what you at going to do. Will cost between £200-£700 a year.
If you are gas safe, factor in the yearly charge.
Tools. Always buy the best. They will last, and pay for themselves over and over again.
Van. You could just get a small van, or even an estate car! Again, it depends on what work you will be doing. A tidy van and you looking reasonably smart will give a professional look. Cost wise, you can pick up a van for £1000. But be careful, as when it's off the road it costs you for the repair and time not at work.
I have been self employed for 20 years. But through health, had a couple of years back on the books. So 5 years ago I stated self employed again. I put £15,000 into my business account and bought the van, stock etc. I then paid it back to myself through the business at £300 a month. Somtimes, if the business was doing ok I would pay back a bit more. ( this was apart from taking wages). In January, I will have been paid back in full.
The main thing you need is work! If you can work mostly for private customers at first. They normally pay straight away. If you are going to work for contractors, you need to factor in that you won't get paid for 30+ days. This won't help cash flow, but once you get past the first month, money should come in every month.
Don't forget e earthing you will earn is taxed! So put a minimum of 20% of your earnings ( not turnover) onto a savings account. Don't think it's yours, or you will be in big trouble when your first tax bill comes through the door.
Before you start, have a word with local sole traders ( builders, joiners etc.) and ask them who they use to do thier accounts. Then go and make an appointment with them. They will advise you on where to start with tax, national insurance and CIS deductions.
Other than that go for it.