Hi, I now live in rural Wales but moved from Birmingham UK.
In the city urban foxes were a major problem and were VERY bold.....we had one who slept on our compost heap and another who would casually stroll up the back garden in the middle of the day. However, the urban foxes didn't bother chickens (or our rabbit) much as they were so overfed on discarded take aways and dustbins. They are, however, unafraid of man and could certainly pose a major threat to your chickens particularly when they have cubs or the weather is cold and they are hungry.
In the country rural foxes are very shy, but hungry as there are no easy food sources, so they continue to pose a problem to chickens.
In cities in GB there are also sparrow hawks who are partial to chicks. In the country buzzards and kites are much more of an issue, though these are shy birds and won't normally come near to the house.
We also have polecats and minks to contend with, but only stoats generally in the city.
Sandie