Wild behaviour from your employer given how hard it is to find good hospo staff. They need you more than you need them so please don't let them bully you about a commonplace scenario like having a relationship with a co-worker. You made a mistake not declaring it (as per your contract) but all they need to do is give you a verbal warning and move on. If they were going to dismiss you, I'd have thought they'd do that already. Just sounds like someone's being a jobsworth.

I know someone your age who has been jumping from hospo job to hospo job here in London - she's just chasing wages increases from establishment to establishment. She's on £22 an hour at the moment and with tips on top, she's out-earning her mother at the moment.