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.
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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.