I am a microlight pilot, and have crossed the Cambridge ILS in the past (non-radio), so thought I would comment on this thread.
What makes you think that a microlight pilot will know what altitude an IFR arrival will be at, on an ILS at 10 miles? I suspect most would assume that an IFR arrival at 5-10 miles would be a lot higher than it actually would be.
I definitely had an ILS pointed out to me on a chart by my instructor during my training, and was told to make sure I stayed well above/below one if crossing it. I was taught to calculate the height of the glideslope at the point of crossing it (c.300ft x distance from runway in nm - I don't remember the exact figure, but would look it up if I needed), and pass well above/below.
From memory on the couple of occasions I have crossed the Cambridge ILS I think I was about 7nm away, and either at 4500ft to be a couple of 1000 ft above, or at 1000ft and keeping a very good lookout for anything big above me that might cause me wake turbulence problems.
This is what I've been taught, and I would expect other microlight pilots also know what the ILS markings mean and where they are. They are extremely obvious on the 1/4 mil charts most of us use.
If it's class G airspace and outside the ATZ it's perfectly legal - stupid but legal.
Would those of you with experience of flying instrument approaches apply "stupid but legal" to any crossing of an ILS, or just one at the approach height? eg. are there break-off or go-around procedures that put traffic crossing one at significant extra risk of confliction regardless of the height?