I only have short action AI's, since I have no where to utilize a long action cartridge (well, until recently).
At any rate, I'm unsure about the UK, but the US you can only buy the AXMC used, or as old new stock. The AXSR replaced it.
I've shot both, and TBH, if I were to buy one today, I"d go to towards the AXSR. AI make an awesome rifle, but do not support older rifles for long, once they have been deprecated by a newer model. Plus the AXSR is just a better design IMHO. It was designed (as I understanded it) to compete for the SOCOM contract. But lost out due to a faulty trigger (which wasn't actually AI's fault).
LR rifles have seen a huge amount of improvements within the past 20 years, so the newer models tend to incorporate that in their designs. AI is no different in that regard. More adjustability, more standardization across the product line, more refinement for modern combat doctrines and TTP's.
JMTCW...