inz4ne, I might be able to offer some basic advice, but I have a few questions first.
If you decided to work for yourself does this mean you would/wouldn't be employed (in the traditional sense) by anyone else?
Have your ex colleagues setup or intend to setup a Ltd Company?
If you decided to work on the project your ex colleagues have asked about, would they be the only people you would develop for?
I sold my software company in 2002, I was a sole trader as this is the easiest way to go when you are on your own, it is certainly very tax efficient, but is only really suitable when you are on your own (hence Sole Trader).
If you setup a Ltd Company then you become an Employee of that Company, when the company makes a profit it pays corporation tax on the profits, you would be paid by the company as an employee paying normal tax and NI, (to minimise the amount of tax and NI you can pay yourself the minimum wage) then as the only shareholder you pay yourself a Dividend which is still taxed, but no NI.
If the only people/Company you are working are your ex colleagues then sooner or later the tax man will insist that really you are just an employee of them and they would insist on you being treated for tax and NI on this basis.
I don't mean to make it sound so complicated, as it isn't really, I would recommend being a sole trader whilst you see how it goes, all you do is contact your local tax office and tell them you are now self employed, they will take your details and your will start being billed for NI (I can't remember which one, but it's not much money) and then you fill in a tax return every year, (I use software to do mine, it's really easy).