The comparison isn't like-for-like: the article is describing sit-down restaurants where diners are likely to spend an hour or more in the restaurant.

A $5 ramen from a chain restaurant in Japan might be viewed as the equivalent of a UK McDonalds meal deal at £5.50 (UK prices generally translate 1 USD into 1 GBP, so it is more expensive in the UK, primarily accounted for by taxes).

You can go to plenty of chains in Japan where you can spend an hour, you'll still spend half of what you pay in the UK.

McDonalds too, a Big Mac meal is £7.99, it is £3.60 in Japan.