Haven't had any such problems, mine has never changed from 60hz, the only time I had a similar problem was when I changed the resolution to 1080p to try something, and when I tried to go back to 4k I was only seeing 29hz option, but turned out that it was nvidia control panel that for some reason has 2 2840 x 2160 resolutions, one in "Ultra HD, HD, SD" which only allows 29hz and another under "PC" which allows the 60hz.
While it is very likely they would do so, it is almost assuredly going to be quite a bit faster if you just go straight to the seller, since you don't need to activate warranty with the seller, they can either replace you the screen (Assuming they have more), or refund you the money.
So I assume I'll have to send this back to Amazon Italy? Would Philips exchange it in the UK under their European warranty?
While it is very likely they would do so, it is almost assuredly going to be quite a bit faster if you just go straight to the seller, since you don't need to activate warranty with the seller, they can either replace you the screen (Assuming they have more), or refund you the money.