Spec-wise, the Boardman has Tiagra compared to Cannondale's sora, plus the Boardman is £200 cheaper (if you're talking about the Cannondale 2017 model), plus extra £20 if you're a BC member.
Should be OK to organise a test ride of the Cannondale at an
Evans, and the Boardman at a Cycle Republic if you have one near (dunno if regular
Halfords accommodate test rides?)