how easy is it to install and american light in the UK?
Issues


Bulbs will be 120V not 240V and will die as soon as connected. In worst case may shower you with broken glass (unlikely but a remote possibility). You will have to fit UK 240V light bulbs instead - they are available with Edison Screw fitting (marked ES or E27).
Fitting won't conform to UK standards. This shouldn't be an issue, legally, as you are not reselling them. 


The kit appears to contain exposed extraneous metal parts that are not earthed - this is regarded as unsafe in the UK and contravenes UK standards. I would drill and tap for screwed earth connection and check for low resistance path from tips of metal parts back to new earth connection.
Kit includes US wire nuts. Not used in UK, probably rated for 120V not 240V. Better to replace with a set of connectors rated for UK use.
Kit includes wire. Insulation may only be rated for 120V not 240V. Might be better to replace with wire rated for 240V.

Is designed to mount to US ceiling boxes - you might need to improvise.
It might be worth checking with your home insurance company whether they will cover fire or accidents related to this fitting.