I just cannot see how this can be a problem, or what this problem can be. You can buy a R/T on any major airline flying between FCO and London, and nobody will care if you throw away the return. No one, and definitely not the UK immigration/customs, who has absolutely nothing to do with this.

I mean unless you're going to fly FCO-London on an airline that you have tonnes of miles with them, and you're going to do with this "throw away return ticket" thing all the time. Then, maybe they will care. But as a one-time deal, definitely not.

And say if you have lots of BA miles, and don't want BA to know you throw away the return, fly Alitalia, or BMI. Or even book BA and just don't give them your FF # - all you lose is like 900 miles or so. No need to use low fare carriers if you don't want to.