Omerch shirt sizes (UK)

Does anyone here have experience buying shirts from Devin's web shop on the UK Omerch site? It seems to be the only site that actually has a variety of Hevy Devy merch, unlike the merch shop on Dev's main site. I'm interested in if these shirts are sized the same as Canadian shirt sizes, or if they are smaller or larger. Any help with this would be great. "When in doubt" type suggestions are welcome, but any real experience through that specific site would be much appreciated.
here's the link, if anyone is unaware of it:
http://www.bandstores.co.uk/shop/devintownsend/
(note sure if board rules allow me to post URLs, but this is Devy related, so I'm guessing that it's fine.)
here's the link, if anyone is unaware of it:
http://www.bandstores.co.uk/shop/devintownsend/
(note sure if board rules allow me to post URLs, but this is Devy related, so I'm guessing that it's fine.)