I’ve had the opposite experience. When I went to London a few years ago I bought a sim card at a vending machine at the airport with a few GB of data. I could add more over the phone if needed. I went to Japan last year and spent a day going to different shops in Tokyo trying to get a sim. All of the stores required long-term contracts and a Japanese home address (I had a similar problem trying to use a cyber cafe since I went without Internet on the trip).
In talking to other tourists, it turns out that it’s way easier to pre-order a sim online and have it shipped to your hotel. The converse seems to be true for London. That info was kind of hard to find online at the time.
