Did you purcgase the phone directly from Samsung or a network @Mariwow ?
This is a similar post > https://eu.community.samsung.com/t5/galaxy-s22-series/100-cash-back-promotion.
I'm not aware of a £100 cashback but if you've seen one then track back to where you've seen it as the claim link and eligibility criteria will be in the small print
Daily Driver > Samsung Galaxy s²³ Ultra 512Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
@Mariwow - Samsung have been running some promotions over the Christmas period so depending on when or how you purchased the device, you will need to check the T's & C's to confirm eligibility.
Based on your purchase date of 8th January, the only promotion that I am aware of running at that time is this one:
Not sure that your device qualifies but you can check. If not, you are probably best going back to Vodafone to confirm and ask them to give you the link to submit your claim.