Extraneous (legitimate) bodies in cabs are a proven cause of distraction and operating incidents, so it is a "big deal" for a TOC to arrange a cab ride. As with any form of risk, the frequency with which the activity is performed is a factor, so the very occasional charity ride might be ok, but nothing regular.
As explained above, there is also a considerable cost involved, as a Driver Manager has to be present. Not only is this taking them off other work, but their time isn't exactly cheap.
Cost - this should raise a couple of thousand, minimum. It is is once in a lifetime opportunity, and there are some rich people with an interest in railways. I saw the GBRF auction, and the seller made a hash of promoting it. He should have had it in Rail, etc. and with a substantial reserve on it.
To give a comparison, the NYMR sell return trip footplate rides for £295. One per day, and they could sell more. However, from the perspective of a driver, they are a monumental pain in the a***. Once someone has paid for something, their attitude can change, compared to those of you above who have been on locos as invited guests. I strongly object to the public in cabs, even in a preservation environment; it is a workplace, and the crew have a job to be doing. I certainly wouldn't be happy about it in the day job.