The motivation from the previous nationalist PiS government in Poland was mainly anything they could to do against Russia or against Germany.
So by supporting Ukraine with military equipment, they were acting against Russia. But at the same time slowed down German support for Ukraine, by blocking maintenance and factories for Ukrainian equipment in Poland. Leading to long transports across to other countries e.g. Latvia for maintenance.
Now, with the grain, this has no Russian involvement and their own farmers are unhappy. So there is no way to go against Russia or against Germany. Also there is a new government in Poland, so I guess we have to see how that develops now.
"Tobias Ellwood, the former chairman of the Commons defence committee" is about half a year behind with his accusation. So either it's just for show or he's not well informed about this topic.
Already October 2023, this was in the news for everyone to read, that France and the UK are providing the geodata for their cruise missile targets and that the UK has personnel in Ukraine.
One example article I just picked via web search: German Tagesspiegel, dated 05.10.2023
https://www.tagesspiegel.de/politik/keine-taurus-lieferung-fur-die-ukraine-erwarte-vom-kanzler-dass-er-endlich-den-weg-freimacht-10572931.html
Quote & DeepL translation: