£270 is the current price for the rubber.
You should be able to call the London boutique and have them ship it (or the AP concession in Harrods will ship)
Entire strap they dont self halves individually sadly.
I can get the measurements vs regular in a bit, since i have both in my spares box.
Edit*
Added some measurements that might help, the holes are spaced slightly closer to the watch on the XS strap,
-so you'd get a 1mm-ish less length using a XS long side (as well as additional super small wrist hole)
-or 10mm less length using the XS short side with the regular long side.
regular 115/75 - (approx hole spacing) 95/87/80/72/65/57
xs 105/65 - (approx hole spacing) 85/77/70/62/55/47