This has been bugging me as I always get confused with the different malt names between different shops & maltsters and end up winging it, so it would be good to get to the bottom of it.
I've been doing some more digging and I've come to the conclusion the Crystal 150 in the Brewdog recipe is referring to EBC or IOB, whereas the Crystal 120 or whatever in the malt substitution chart you posted (which is actually one of the ones I had looked at and got confused) is referring to Lovibond.
So I think I got lucky again as the Medium Crystal I chose looks about right at 130-150 IOB, 140-160 EBC, as I believe 120L would be much darker at over 300 EBC.
Some of the other recipes in DIYDOG call for Dark Crystal 350, which I think is too high to be a Lovibond number, but looks to be in the correct EBC/IOB range for Dark Crystal.
Here's some links I've been digging up:
Thomas Fawcett (EBC):
http://www.fawcett-maltsters.co.uk/uploads/2/0/2/6/20260333/spec_table_ebc.pdf
Crystal Malt = 150-175 EBC
Thomas Fawcett (Lovibond):
http://www.fawcett-maltsters.co.uk/uploads/2/0/2/6/20260333/spec_table_asbc.pdf
Crystal Malt I = 40-50 Lovibond
Thomas Fawcett (IOB):
http://www.fawcett-maltsters.co.uk/uploads/2/0/2/6/20260333/spec_table_iob.pdf
Crystal Malt = 120-140 IOB EBC
Bairds:
http://www.brewersselect.co.uk/document/38
Crystal Medium = 140-160 EBC
Muntons:
http://www.muntons-inc.com/craft-brewery-products/
Crystal Malt 150 = 130-170 EBC
Briess:
http://www.briessmalting.com/Assets/PDFs/Briess_TypicalAnalysis_Flyer.pdf
Interestingly they don't have anything called Crystal, but do have Caramel Malt 120L = 120 Lovibond = 318 EBC (according to this calculator
https://www.brewtoad.com/tools/color-converter)
So I now have a better understanding of why I was so confused when looking the malt substitution chart, but it is still useful so long as you know that any figures are in Lovibond.
Hope that helps someone