- Are there any particular reasons why this idea is not advisable?
No, no reasons why not. It's the legal way to do things.
- How would health care in Poland work as a self-employed?
Same as an employed person - you're covered on a month-to-month basis, as long as you pay the contributions. All you have to do is print off the proof of having paid the social insurance when you use public healthcare.
- In case I would fall pregnant how would health care be then in Poland or would this be through my husband anyhow if he works in Poland?
If you fall pregnant, then you are entitled to maternity leave/medical care as a self employed person. But if you stop being self employed, then as long as your husband pays social insurance, you're covered under him.
The only real issue is that your taxes *could* be higher in Poland - it depends on exactly how much you're earning in the UK. But you should be in a position to ask for a pay rise as you'll be covering your own social insurance from now on - which should negate any higher taxes in Poland.
As for taxation for a self employed person in Poland - 18% up to 85k a year, then 32% of anything over 85k. Plus 360zl a month compulsory social contributions (including health care) - rises to 900zl after 2 years. The other advantage is that you can dock off a wide range of things off your tax bill - allowed deductions here are far more liberal than in the UK :)