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I fairly certain this is a tabulate (colonial) coral as you originally thought. The internal transverse septa along the tubes are characteristic (crinoids and worm tubes wouldn't have these). Also I agree with your suggestion that it is probably a glacial erratic. The lithology and the fossil and the location should allow a more definitive conclusion, but I'm not a geologist either so I will pass on that!
We do not have a separate category for fossils (or expertise in same) at the moment, so let me just tell folks that Peter Skelton is a professional palaeontologist (& until very recently an academic at the OU). See his ID, above.
University of Edinburgh and Biodiversity Observatory (OU)
It is a fascinating object just by itself but so much more so now I know a bit more about it!
Lat/Lng: 54.986, -1.651
OS grid ref: NZ224657