That sounds really difficult. I mean, what you probably shouldn't do (if you can possibly avoid it) is eat fish 'n' chips. It's really not good for your body to 'jump' in and out of VLCD-mode - and psychologically I can't imagine eating food one day and products is the next is very helpful either.
So with my most practical hat on I guess you've got ... three choices?
1. If you feel you can go and resist the temptation to eat (although the smell of fish 'n' chips would kill me, honestly) and you want to see them before you go on holiday: then go. If you do go, you should probably explain in advance that you're going because you want to see them but you won't be eating and ask how they feel about that (after all, it's socially awkward if person isn't eating, and my ruin their tea): you can explain the not-eating by saying you're on a diet and asking for their support (if they're not supportive of you, that's a different level of problem) or if you're okay telling a white lie to your family, because you don't want to get into a diet talk with them, you could say you're not feeling well (tummy trouble? nobody ever asks about that
) and don't want to eat something so greasy and heavy.
2. You can cry off: either by telling the truth that you're on a diet and you feel being around food will be hard or, again, a white lie and say you're not well.
3. Alternatively you could use either 'excuse' (truth or white lie - you're on a diet, you're not feeling well) and suggest you come round to see them AFTER they've eaten. After all, presumably they want to see *you*. Not see you eat fish 'n' chips. If that makes sense.
Hope this helps. I definitely don't want to tell you what you to do, and only you know your family, your relationship with them and how they're likely to react in a given situation, but my strong feeling is that you *shouldn't* feel bullied / socially pressured into eating fish 'n' chips. The thing is, going on a diet is something you are doing *for you* - for your health, comfort and self-esteem. It is okay to do prioritise something you are doing for you over the feelings of other people occasionally. Because, as much as they love you and you love them, your body is your business not your family's. And if they are uncomfortable with you doing something for your own health and peace ... well ... as I said above. That's a different level of problem.
Good luck!