Chris, the SPS-410's are a 4 point mount as well - that's fairly standard. Evl describes using a big flat washer to clamp the speaker frame where you fabricated a square one.
Might be a difference in the shape of the speaker frame, just like the lugs can be a different shape and 1 needed cutting off in your case.
Not sure about your point on the rear speakers - maybe it has been discussed that although both pairs of speakers are well worth upgrading, you will notice the effect more in the front.
Because your model doesn't have the factory component tweeter in the fronts that the N. American market base system has, you benefit even more with the upgrade to co-axials.
Don't know what your best option would be on the amplifier situation - just surprised that the base U.K. system doesn't even have one, especially as the stock 4" door speakers look the same.