I will still be working full time for around 3 months in the UK after they move to Poland then 3 days most weeks in the UK and weekends in Poland, I will be in the UK more than the 183 days so i assume my tax will all be paid to the UK?
Quite possibly not. For a start, if you're only in the UK for three days per week, you'll be in Poland for four times fifty-two days, which is 208. For another, the 183-day rule is not actually the applicable rule in Poland. 183 days or more (or fewer) in the country is only part of the test Poland applies, the main factor is where you 'centre of life' is (I'd be guessing that for you it's going to be Poland).
The plan is for me to move full time to Poland although i assume until i can speak a good level of Polish my possible job options would be very limited?
To work in the sector here you'll need to meet the minimum qualification standards (which you might well not meet) and have your UK qualifications recognised as equivalent to Polish qualifications (which the relevant bodies may very well not want to do). To give you one example, in Poland there are 12 years of school, then five years of post school education (all five of which can be done part-time, as can the last three years of school) and you have a Magister; in the UK you do 12 years of school and then at least five years of post school education (two years of college and three (at least) years at university, none of which can be done part-time) and you have a Bachelor's degree. But Poland insists that a Magister is a higher level than a Bachelor's degree.
English teaching seems what most people initially do, how much does this pay on average in a town with little or no competion from fellow British etc?
Depends how rich or poor the town is. If you have no qualifications or experience, probably a couple of thousand zloty a month, four thousand at the most.
Will i be able to apply for child benefit and any child tax credits while still working in the UK?
Yes. That at least is some good news for you.
I assume i would carry on using the EHIC card until i moved over full time.
That is only for emergency treatment as a tourist. And some Polish hospitals very simply reject it anyway.
We will have no major housing costs as we will be staying with her family to start with and will be converting empty property on their land.
That conversion sounds like it's going to cost a fair bit. And that's assuming it is just converted. A bloke I know in Warsaw was talked by his wife into paying for summer house for them to be built in the field outside her parents' house in the small-ish town she's from (a four-hour drive from Warsaw). It went from an 80-metre summer-only house to a 350-metre year-round house which she now lives in all-year round.
Sorry to possibly speak out of turn, but have you thought about what you're doing here? You really want to move to a town where nobody speaks your language while you don't speak Polish? Rural life can be isolated enough, especially if you don't have family and/or friends around you, but factor in not speaking the language or having a job and it's certainly not something I'd want to do.