Many thanks to those who have replied so far.
The point that Hartington makes regarding clearing Customs and Immigration at Denver is something that I did realise would need to be done but the pre-flight checks that I was referring to are the ones where, in previous experience at GLA or EDI, the ICTS people have turned up at the check-in desks with their laptops etc and gone through all the various questions before you even reach the check-in desk. The question I'm interested in having answered is whether these ICTS checks (or similar) will be done at GLA or at KEF, that being the point of embarkation to the US. Apologies for not making the question clear in my first post.