Hello Ben,
I won't give specific advice about which car to go for, but over the years both the successive models of Astra and the Focus have grown in weight and size. This has put extra strain on the engine options, Some consider the smaller versions are under powered. You must check to see if the car you consider is actually rated to pull the 800Kg caravan - some of the smaller Focuses are only rated to 700Kg, and the Astras to 750Kg
Towing a caravan is adding a considerable extra load to the car. Small engined cars towing caravans will loose more performance than bigger engined cars, so I suggest you look at the larger engines.
May I also suggest you consider a diesel engined variant. The power delivery from a diesel is usually better suited to towing than the high revving petrol engines, and the fuel economy when driving solo can be quite impressive.
I did have Focus 1.8 Zetec petrol Estate for a while. I did not tow with it, but even solo I was disappointed with its fuel economy. I had previously been running an elderly Saab 9000i 2.0litre which had better economy than the Ford, despite being bigger and heavier!
Both Astra and Focus have won awards for their designs, and I am not aware of any significant short commings with regard to reliability for either models. My brother in Law who runs garage reckons that spares for Fords are perhaps fractionally cheaper but there is not much in it.
Apart from the fuel economy issue I was not happy with the support I got from the Fords seating. This is a very personal matter, so I suggest that you really evaluate the difference in the seating carefully.