And the 2 year thing is wishful thinking - the IR can look at you at any time and decide you are employed based on how you work. If you are at the same desk in the same office for 40 hours per week, and you have to ask permission for holidays and you have to do whatever you are asked to do then you look like an employee even if you have only been there for a few months.
The reason most contractors in the IT/City have been limited companies is not the tax benefits (you are better off self employed) it's because the clients don't want more staff on the headcount, so they can't take on employees. They won't take on self-employed as if the IR decide you look employed rather than self employed the CLIENT is liable for unpaid PAYE/NI (based on what they pay you as the net). If you are through a Ltd Company you (your Company) are liable for unpaid tax/NI (IR35). Although I'm not sure whether anyone who has been caught by IR35 has taken their client/employer to court and claimed employee rights? You'd never work again so there wouldn't be much point.
Tax wise IF you need (say) £50K pa to live on, then you are about £4K pa better off by taking it through a company, with the minimum as salary and the rest as dividends (that's the saving on NI that you get by taking dividends). There is no tax benefit in taking the income as divis other than that. UNLESS you can a) save money in the company and take it as income in a year when you have less money coming in, so stay in a lower tax band or b) can split the company income between multiple employees/shareholders again to keep you both in lower tax bands.
In the case of child benefit you almost certainly could keep your income under the limit by use of expenses, or by giving a small shareholding to your partner so that it counted against their income instead. But you do still have to pay for accountants/ company filings etc, and you do lose all the employment rights (maternity pay, dismissal etc as well as pension contributions/holidays/sick pay).
There are better ways to keep your income under the limit for CB (bump your pension contrib by salary sacrifice) than to become a limited company.