Hello Phil, what an interesting question. There probably is some list but I doubt if it is available to the public. However, the information (if I understand your question correctly) is there for the asking via the advanced search feature on the TNA Discovery catalogue.
Go to 'advanced search'; find 'Search for or within references, Any of these references' near the bottom of the first field; enter the file series you are curious about, such as AIR29;
... hit search and you will find that there are 9,593 individual files at Kew in the AIR29 series.
I know the search shows 9,594 hits but look to the left and you will see that it has caught one file held at another archive. It works with any series of files.
If I have missed the thrust of your question, my apologies and please let me know what needs tweeking.
Hope this helps.