As the UK and PL are in the EU there would be double taxation relief meaning that we would only be taxed in the UK (corporation tax of 20%) and cannot be taxed again in PL.
No, not quite. You're missing the vital dividends part. I'll try and explain :
Company makes a pre-tax profit of 50,000GBP
Minus UK corporation tax of 20% - leaves you 40,000GBP in profits
For the sake of argument, you pay yourself 40,000GBP in dividends.
Now, the money is transferred to Poland, so let's use the figure of 218596PLN.
You take off tax free amount (I'm using 6200PLN as the number as I don't know what the married allowance is nor the allowance for kids)
218596 - 6200 = 212396PLN
You then pay 40355PLN (19% of the dividends after the tax free amount to the Polish tax office, leaving you with 171056PLN or 31293GBP. It's an effective tax rate of around 37.5%.
In regards to double taxation - all it means is that the same income isn't taxed twice. If you pay (personally) 2000GBP cash to the British HMRC, then Poland will give you a credit for the 2000GBP cash paid.
I would get these numbers checked by a professional, but I think they're broadly correct. The thing I'm not sure about is how a Polish branch office affects things. I'm pretty sure you can lower the taxable amount by quite a bit, but you'd need specialist advice on how to handle it.