No they're not. The ballot papers themselves are indistinguishable, and they are rejected during counting if people include any identifying marks.

Are exit polls not anonymized in the UK? At the last federal election in Germany, I was asked to contribute in an exit poll, and they had their own voting booth and urn set up for that purpose next to the actual voting room.

The ballot was mostly identical, except for additional checkboxes for age group and gender, which they wanted to break down in their analyses. So still reasonably anonymized.