I went to https://osdatahub.os.uk/ and signed up for a free account, but have no idea where to find an API key.

Okay, there is a description of the process at https://osdatahub.os.uk/support#generateApiKey and I managed to make one – I would recommend testing this on some new users. These instructions are tricky to find, and the flow for creating an API key is a bit unintuitive.

When there is no API key entered above, you may want to throw an error with a descriptive message telling someone they need an API key (or even output an HTML element with a message like that), instead of failing with some kind of internal syntax error.

+1 I totally agree, it says at the end of the notebook. But how to update the apikey?

Went through the troubles of creating an apikey, but it was no use, still got the same json parse error.

I went ahead and updated the API key, it's been a while and I think mine had expired. I'm working on getting the map tiles to load now ...

Unfortunately hitting snags getting the map tiles to load along with the vector overlay — I'll revisit this soon 🪄

You may want to round coordinates a bit, cf. https://xkcd.com/2170/

Thanks for the tip, did it :)