Ketchup in UK

I'm writing a Torchwood fanfic and need to know about ketchup on chips. I have Mica Davies and Jack eating at a McDonalds and they're both eating ketchup on their chips.

Is this believable? Should I have Jack eating them with malt vinegar or something? Would a six year old actually do that or have they been indoctrinated to want ketchup at McDonalds?

Please help!