To add to what Kimonas said, if you are running a business you must register with social insurance.
Many Brits have been operating illegally, trying to get away with not paying but the social insurance office are clamping down hard and the penalties for any who is caught are severe, we know of brits who own property here who have been deported.
Having said that, if you are paying into the social fund you are entitled to free hospital treatment so you don't need private health insurance, also it goes towards topping up your old age pension.
If you deregister for tax in the UK and pay tax over here you can earn far more before you pay tax. You can earn up 19.000euros before tax and after that its 10% for the next tax bracket.
If you have a business in two names you both get a the allowance so you can earn almost 40.000 before paying tax.
Veronica