Since they're sending the request some days prior to return travel, by which time many/most people already have their tickets printed out (and, if already on the move, might not see the request before turning up for their return train), then it's clearly not enforceable. UK border authorities clearly need to ask Eurostar to include this information collection as part of the ticket buying process; until that happens, it can't be retrospective.
Also, I note that the request sent out asks for information taken from the passport to be used for the journey. Some of the info is exactly what's on the passport - number, issuer, etc - but one item isn't. UK passports show a person's "sex", whilst the request refers to the person's "gender". Since these are separate concepts, and the answer might not be the same to each question, this is rather weird. Is this a howler perpetrated by someone at Border Force asking Eurostar to do this? Or (more likely I guess), is it someone at Eurostar misinterpreting/miscopying the request (maybe whilst trying to be very "up to date" and not thinking through the logic of the situation)?. I foresee some hoo-haa...