We sometimes forget on this forum the majority of visitors to public aviation attractions are casual, not entheusiasts!!
There around 60 aircraft museums in the UK. If we want to attract “new blood” into our hobby they need something to inspire them, unfortunately most of the UK’s “desirable” and iconic aircraft that the public know are found in a handful of museums and collections. Those that have “iconic” or valuable aircraft in store understandably do not let them go to other collections.
So the if smaller collections want a particular aircraft their only remit is to build or buy a Full Scale Model.