If it is a UK charity, is unlikely to be governed by Polish law.
The website may be able to rely on their terms and conditions but only if their terms and conditions were brought to your attention at the time that you posted the work online. You will have seen the online websites where you buy things, you have to tick a box to confirm that you have read, understand and agree to their terms and conditions. Some websites don’t actually have that but they have a very prominent message by the “proceed” button to the same effect. If their terms and conditions are hidden on the website and you didn’t know that you are agreeing to this (not reading the and not understanding them is not the same) then you have given them permission to do with it as they wish. In that case, there would be nothing you could do about this.
Assuming that these terms and conditions were not prominent or you didn’t have to do make a positive decision to accept them, then they use the work under a licence and the nature of a licence is that it can be terminated on reasonable notice.
It depends whether this charity would go to the ends of the earth to keep the work on their website and whether, if you threaten to take them to court, or actually took them to court, they would spend money in defending the proceedings because there is always a risk that they would lose and you would get costs awarded against them.
At present, they are ignoring your requests but they may not ignore a letter from a solicitor threatening legal proceedings if they do, they may not ignore the actual issue of legal proceedings which would be for an injunction to make them remove the work. The court would have to decide whether you had given a lifelong agreement or not.
On a practical basis, you say it’s a UK charity but I don’t know if they have a presence in the UK. If they only have a physical presence in Poland and the terms of the website and posting things online are indeed under Polish law that you will have to issue legal proceedings in Poland with all the problems that entails. You would need to speak to a Polish lawyer about how to do that. That is going to be complicated, potentially risky and certainly not cheap.
Can I clarify anything for you?
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