Only one I can think of is Auto Italia magazine. Not specific to Fiat, as its title suggests, but does cover Fiats occasionally. Should be in WH Smith.
auto italia is a good read on anything italian (funny enough)as well as ferarri lancia alfa and maserati this months edition has a 6 page feature on the 500l and good feature on fiats first entry to world rallying with the fiat 125.
I asked this question because, after having spent quite a few hours looking on the internet, I came up with nothing. Thanks portland_bill, I shall have a look next time I am in there.