I don't know for FCO, but in several other stations BT has never weighted my carryon nor had a problem with multiple ones (usually a laptop bag or similar and a rollaboard plus of course duty free shopping bags ). Just be sensible and discreet about it, don't act like it weights a ton and definitely don't bring it up.

I doubt any agent willingly wants to go through the hassle for charging you overweight for half a kilo - that much could be accounted for a miss calibrated scale for example.

Oh and if OLCI is available ex-FCO use it to avoid contact with the check in personnel and a chance for weighting.