Films

I caught I Daniel Blake and wouldn't describe it as Loach's best work

I would probably agree, but then I haven't seen all of his films, with some notable gaps, so I wouldn't know. :)

and was left wondering how it won at Cannes.

Most likely because all the rest were not even close.

It has some important things to say (how unemployed are mistreated and marginalised)

I think it not only says them, but expresses them--I can't stand films that just have something important to say. :)

but film felt rather 'constructed' in a way that was trying to make a point rather than reflecting reality

I agree; as I said above, to me the main weakness was in the script, particularly the exposition (or lack thereof, as usual), as well as some poorly-thought out transitions in which the causes were not clear enough. At times you didn't really understand why Daniel acted in the way he did, and that always weakens a film, especially here, where it leaves you wondering if there really was no other route for him to take. I also think the climactic scene was poorly prepared.

I still think that it's a very good film, because despite the above weaknesses I think it's a good script (only one or two lines that were delivered slightly awkwardly and didn't quite fit, generally good and natural dialogue), because of the acting (Ken Loach is simply excellent at working with actors), because I like his directing style, and because it is simply very moving, honest, and direct.