Hello Foopaitass93
The advice from our SBB colleagues is absolutely correct.
This service is no longer offered for outbound flights to Switzerland. And the UK travel agent offers it exclusively for their customers.
You could take your luggage yourself through customs and check-in at the SBB travel center for further transportation. This means, however, the luggage will only arrive two days later at your destination.
https://commerce.sbb.ch/en/luggage.html
Kind regards,
Roland