429 code is the Throttle response - Maybe more Throttle responses / checks have been added
Interestingly I just checked this API and it always returns items per page as “100” on anything over “100” if you use start_index=0 and items_per_page=1000 you do not get error 416 (But still shows items_per_page as 100) but put start_index=1 and items_per_page=1000 and you do get error 416
This was for the query https://api.company-information.service.gov.uk/search/companies?items_per_page=1000&start_index=0&q=World
"kind": "search#companies",
"page_number": 1,
"items_per_page": 100,
"total_results": 40841,
"start_index": 0
and the same for https://api.company-information.service.gov.uk/search/companies?items_per_page=500&start_index=1&q=World
"kind": "search#companies",
"page_number": 1,
"items_per_page": 100,
"total_results": 40841,
"start_index": 0
So it adjusting my start_index from 1 to 0 and showing items_per_page as 100