Hi Stuart,
I tried changing 'Display H5P' - 'Allowed sources' to 'https://elearningworld.h5p.com/content/[id]/embed' and adding 'https://elearningworld.h5p.com/content/1290877969109473569/embed' text to a page, and it seems to work OK.
Just notice that because MDL-67337 has just landed (actually, will be landing it this week, so officially hadn't landed yet
) filter is not applied inside a link. And now when you copy a URL from the URL bar in Chrome and paste in an editor, is pasted as a link. So that would avoid the filter to be applied. Could you please confirm that the content URL is not added inside a link tag? If that's the case, you could remove the link tag and leave only the text itself with the URL or wait to upgrade 
I tried changing 'Display H5P' - 'Allowed sources' to 'https://elearningworld.h5p.com/content/[id]/embed' and adding 'https://elearningworld.h5p.com/content/1290877969109473569/embed' text to a page, and it seems to work OK.
Just notice that because MDL-67337 has just landed (actually, will be landing it this week, so officially hadn't landed yet
Otherwise, could you please check that 'Display H5P' filter is on and upper than 'Convert URLs into links and images' filters?
I hope this helps.
Amaia