All GPS Satnavs will have a problem when surrounded by tall buildings ; I suspect that is all that was being referred to. An external aerial may improve matters a little but there is no catchall solution - for an i3, Tomtom or any other make.
If the antenna can't "see" the satellites then it can't get a location fix - simple as that.
I live in South London and have had no problems in the few days I have had one so far (It gets a signal in seconds of turning on) I am off to central London today and will let you know how it performed.
Driven from East London to the City of London, then through Trafalgar Square on the way to the Natural History Museum and the i3 lost it's signal about half dozen times. Each time it lost the signal it took less than a minute to lock onto again.
For the price I cannot fault it and can say that I have had other sat navs and they all lose their signal occasionally on the journey I described above.