Following Badlad's comments about the Department for Digital, Culture, Media & Sport making a specific reference to caving, I thought I'd see if I could find this or anything else useful anywhere in their published documents.

I didn't find it, but I did stumble across a Government page relating to "Adventure Activities Licences" which I'm sure many people on this forum will be familiar with, however I am not, so I will share the text from the Government website:

Contact the Adventure Activities Licensing Authority (AALA) if you want to charge for outdoor adventure activities for under 18s.

You?ll usually need a licence for the following activities if they?re done in remote or isolated areas:
- climbing
- watersports
- caving
- trekking

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Full context available here: https://www.gov.uk/adventure-activities-licence

Further guidance is available from HSE in the book "L77 Guidance from the Licensing Authority on the Adventure Activities Licensing Regulations 2004" which states:

The aim of the licensing scheme is to give assurance that good safety management
practice is being followed so that young people can continue to have opportunities
to experience exciting and stimulating activities outdoors.

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Full document available here: https://www.hse.gov.uk/pubns/priced/l77.pdf

So for the doubters out there, caving is officially outdoors. 

Brilliant ,try telling Graham Mullen that !

Other than for mischief or used to support their own issues, did anyone really think otherwise?

The Welsh Government? DarknessBelow?

Oh wait, sorry, you said other than to support their own issues...  :-\

Forget that then

Apologies for being a wet blanket but unfortunately, CRoW uses the phrase "open air" not "outdoor".  And when you get into the detail, the Activity Centres (Young Persons? Safety) Act of 1995 defines ?facilities for adventure activities? means such facilities, for such sporting, recreational or outdoor activities.  The Adventure Activities Licensing Regulations of 2004 goes onto to define ?adventure activity? means caving, climbing, trekking or watersports.  So there is a lack of certainty in the definitions used in law to link caving to being an outdoor activity which a good barrister will use.  The fact that officials may suggest otherwise does not overrule what the acts and regulations say.

Agree that CRoW refers to "open air" not "outdoors" however the term "outdoors" has been used in a number of other scenarios if I am not mistaken, particularly recently in relation to exercising during the lockdown.

Not sure I agree with you about the certainty of the regulations. The guidance makes clear the intent and you would be hard pressed to argue against that, however

ultimately only the courts can decide the effect of the Regulations

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& also depends how junior the member of staff who put the letter together is.

mikem said:

& also depends how junior the member of staff who put the letter together is.

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Which letter Mike? The official guidance I referenced is a published HSE book (first published 1996, 2nd edition 2007). It is absurd to suggest that this would have been produced by a junior member of staff and not checked. Link below again for reference.
https://www.hse.gov.uk/pubns/priced/l77.pdf

It's amazing how frantic some people become when someone finds another reason for caving being an outdoor sport.

A surgeon friend once told me to remember that when I write a letter then I am writing a letter to the judge.  I guess he's had a close call or two.  Anyone who writes letters on official letterhead needs to remember where they might end up.  Likewise the minutes of meetings.

Lawyers have all the time in the world to pick over the carcass.

I was referring to Badlad's letter that motivated your search....