Well, I was also a patient Londoner who would take some pains to make sure confused tourists got where they were trying to go, but I always found London a fairly friendly, if brisk, place, in all the time I lived there (not from the UK) -- no one is necessarily going to be smiley or chatting at bus stops, and people practice the mental tricks needed to survive crowded journeys on public transport, but always helped with luggage or a pushchair on stairs in the tube, and I was always offered a seat when pregnant. But clearly to some visitors, that constitutes unfriendliness.
But think about your point about Slovenia and Croatia. Isn't it really more likely that this was just the small selection of people you happened to meet in both countries than that an entire population was surly or friendly? I do think there was a lingering lack of 'customer service' in some post-communist countries, because I noticed it in Hungary, where I spent a fair bit of time in the 90s, and my BIL, who was managing waiting and bar staff while working there, said the same, but I would have thought that would have long dissipated.
I think there's also a level of understanding what constitutes friendliness or rudeness in a particular culture. I remember an encounter in a Parisian supermarket years ago. An American tourist walked up to an employee and said 'Mayo? MAYO?' getting louder and louder as he thought he wasn't being understood. The employee said neutrally 'Bon jour, monsieur.' The tourist just kept shouting 'MAYO!' and getting increasingly irritated, and the employee, clearly irritated, kept prompting him to say 'Bon jour'. I've no idea what actually happened and if the mayonnaise ever got bought, but the same employee was very helpful to me when I needed to find something a few minutes later. In part, probably, because, although my French is rusty, I know that you greet staff on entering a shop. My point being that both those people will have gone away thinking 'Rude Parisians!' and 'Rude Americans!' because of their different cultural norms.