UK resident, working for Polish company remotely - how to prevent 20% being deducted in Poland?

Hi, The company which I have just started working for, have asked me to provide a certificate of tax residence, in order that they can pay me gross, and not have to deduct 20% tax in Poland. I filled in the form to generate a Certificate of Residence. I received a confirmation of residence, which only confirms that I was resident in the UK for 1 week this December. I'm not sure why it only covers one week. The company will not accept the confirmation of residence, and point to the fact that it says on it 'This is not a certificate of residence for the purpose of claiming benefits under any double taxation agreement with the UK'. I called the HMRC, and asked what do I need to prevent paying this 20% tax in the first place, since the certificate of residence only seems to be issues retroactively for money already earned, and the confirmation of residence only confirms my residence in the past. The agent I spoke to, did not know how to advise me, and I'm he could not get through to a 'technical advisor' as none were available, so I'm waiting for a call back, which could take up to 3 days. Can anyone advise, on what document I'd need not to avoid paying tax in Poland, on income earned while working remotely in the UK, and for which I'll be liable for tax in the UK at a future date? Thanks, Steven

Hi,
Please request a certificate of residence, by submitting the form at How to apply for a certificate of residence to claim tax relief abroad.  
Please use the online service for the fasted response.  
You will need to ensure that you declare the amount of income arising in Poland.  
As this is employment income it should be declared in the employment section of the tax return (SA102 on paper) Self Assessment tax return forms.
Thank you.