Interesting idea, but there's still the problem of a massive scaling-up operation. Do we know whether the Orion constellation as a whole had any significance to ancient Britons or was it just the belt stars? If the former, are there any other sites in the area that align to the other stars in the constellation? That would give much more weight to the argument.
In the case of the Giza pyramids, the sizes and alignments can all be attributed to practical considerations of construction. There is also a significant error in the supposed alignment that Hancock tried to use to project the date of construction back to 10,500BC by consideration of the Earth's precession, which I regard as clutching at straws.
There is a great tendency to assume that an alignment of three is highly significant, but this is not necessarily so. The first two can be scaled and aligned to anything, so it's really only the relative position of the third that has any significance. On the other hand, if an alignment was not intended then it would be more likely that the henges would be in a straight line, unless there is another more practical explanation for the deviation.
