Alex mate, may I offer congratulations and a couple of corrections please?
The BC1 came first as a development out of the BBC research carried out by Spen Hughes, Dudley Harwood (Harbeth) and other notables. The "official" BBC product, the LS3/6, apparently didn't sell to the Beeb in vast quantities, yet the simpler (less adjustable crossover) derivative from this research, the Spendor BC1, sold in large numbers right up until the early 80's I understand, finally being taken out of service in 1987 (from memory).
The BC1 idea was developed a little for better bass and higher power handling with the BC2 which I have, the trade-off being lack of absolute smoothness at the crossover point, a slight suckout being introduced to mask this. Only really an issue if they're too close to a wall, which lifts the bass even more.
This neutral to subtle upper mid reacts severely with the peaked up and phasey crossover region and fizzy treble exhibited by so many "Flat-Earth" speakers like many early Royds and Kans for example, the "BBC" way of doing things making the Spendors and Rogers of this period sound a little boomy and recessed - boring in other words - One reason so few flat-earthers listen to full-on orchestral or acoustic music in general I found personally.
As the 70's went on and the BC1 went through a much publicised driver-surround issue, the new Polypropylene wonder-cone came along. Spendor developed the SP1, which has 90% of the BC1's midband lucidity and a further improved bass quality and power handling of the BC2. By the launch date of 1983, my experience was that Rogers had taken over this end of the domestic market, the various "Studio 1" models being nicer looking and very popular, despite a bass heaviness quite often.
For me, the SP1 was a glorious return to form. These worked a treat with Naim (important back then) and the higher open Target stands were often used with these. The BC3-esque styling was much more up-to-date (my old BC2's are really looking tired now) and the tightly fitting black grille-cloth didn't sag as mine have now the sponge underneath has crumbled away. I last listened to some in anger a few years ago and they more than held their own easily over significantly more expensive alternatives I thought. They just have a great blend of virtues and cover their faint tracks really well.
These days, Spendor don't seem to want to vastly promote their "classic" range and put it about that the S, SE and I suspect the A range is superior. I cannot possibly judge this, except to say that Harbeth have stepped in and can always pull a pair of their excellent speakers for UK sales from an export order as necessary.
One final thing and then I'll shut up and return to my studies, which keep being neglected for audio reasons
people who bought and buy "BBC" type speakers tend to keep them for decades (Tannoys too I believe). If the particular recipe suits, the amount of enjoyment gained is enormous and very long lasting. be warned though, old BC1's, 2's and 3's bass/mid are no longer supported by Spendor and a blow-up will mean either scrapping the speakers or buying another pair for parts, which may not match the crossovers properly. I believe SP1's may still be supported BUT.. SP1/2, SP2/2 and it's siblings from the late eighties onwards may suffer stiffening of the rubber-compound surrounds (ooohhh missus) as has happened with some SP2/3's I know of that were little used in an editing suite since the late 90's.
I can't go on and I can't go back, I don't feel so matter of fact,
I tried and tried to make good sense, What's the good to try it all again?