A few possible upgrades from Spendor BC-1

I would add the Spendor 2/3 speaker and the SP100 to your list.

If your uncle has room the Green Mountain Continuum 2s currently for sale on Audiogon look like a great deal. Visually some people find them unacceptable but they sound good and are adjustable for different listening distances.

I know most of the speakers you list although I have never heard the BC3. The SP1/2 is the closest to the BC1. Some listeners find it less "magical" than the BC1 but it is actually significantly more accurate. It tightens up the slightly loose bass of the BC1 considerably although I still wouldn't call it perfectly taut. It also has more power handling and will play louder than the BC1 but it is still not comfortable at really high volume.
The Dynaudios have never excited me. They are quite neutral but I find them uninvolving. Many of the Dynaudio models seem to like big SS amps.
I haven't heard the Harbeth SHL-5 but I am familiar with the M30 & M40. The M30 is more resolving than the SP1/2 but doesn't play quite as deep. The BC1 and, to a lesser extent the SP1/2 are voiced slightly on the warm side of neutral. The Harbeth sound is cooler and they are less forgiving of aggressive amps than the Spendors. The M30 and M40 need more physical space (i.e. a larger room, more distance from boundaries) than the SP1/2 to sound their best. Like the Dynaudios they seem to sound better with larger SS amps.
I haven't heard the Green Mountain Callisto but based on several other Green Mountain speakers I think your uncle would like these. I suspect they are more resolving than any of the Spendors but less analytical sounding than then the Harbeth M30s and more musically involving than the Dynaudios.
The Spendor BC3 had a reputation for being tough to drive. As I noted I haven't heard them.
One more possible option might be a used pair of ProAc Response 2.5s or maybe even Response 2s. These also have sonic character somewhat in the ballpark of the BC1.
Without knowing more about the room and electronics I'm afraid I can't offer any more advice. Good luck.