BCA has overlooked the advantages of seeking relationships with other like minded organisations for most of its history. To some extent the problem has been a lack of volunteers to effectively forge those relationships. The other significant problem is that neither the executive or council have ever really bought in to doing so. From my experience BCA prefer to look inwards rather than outwards.
To me it has always been a no brainer. Many sister like organisations are far larger than BCA with much larger resources, paid full time staff, etc. That can be really helpful and save duplication of effort in many things. Working out how new legislation might effect clubs huts for example, are one of many common issues many associations face. If campaigning for rights, recognition from government, general lobbying for outdoor sport, then relationships with organisations with more resources can be enormously helpful. Without them the BCA voice is so small, or non existent that it isn't even heard.
During the three years I was BCA CRoW Liaison Officer I established relationships with the organisations listed below. This included face to face meetings, phone conversations, participation in mutual campaigns, etc. I visited Westminster on behalf of British Caving. Took MPs caving and generally was in touch with as many people who were in a position to support caving as I could find. When I stood down I don't think anyone really maintained these relationships and BCA slipped into obscurity again.
British Mountaineering
British Canoeing
Sport & Recreational Alliance
Outdoor Industries Association
British Orienteering
Land Access and Recreational Association
APPG on Mountaineering
Campaign for National Parks
Others I have probably forgotten.
Building these sorts of relationships should be a high priority for our national body but it just isn't. I doubt very much that BCA has joined 'Outdoors for All' to answer your question. Not only won't it have joined, it probably won't even be known to BCA and therefore not even discussed at council. Worse still, I expect BCA is so low on the radar of other organisations that no one even thought to invite BCA to join, that is a sad state of affairs I reckon.