"I do consider it rude" So do I, but I wouldn't label someone as a 'rude person' if it isn't the totality of their behaviour.
Whitesplaining. - I cannot speak for people who have been whitesplained to, but I imagine the times I am most likely to lapse into it myself, would be in a group round-the-table discussion where I voice my opinion way more than everyone else, if I am the only white person in the room.
I often notice if I am at, say, a mothers' post-natal group having a big discussion and there's only one WoC in the room (particularly not native British), she will usually be quite quiet in the mix. I fear that there is a lot of whitesplaining going on at that time to make her feel uncomfortable about joining in.
Ways I think I am far less likely to do it, is to speak of my cultural norms as the norm, and the 'rightness' of these norms, or to act like my time in Africa qualifies me to speak about what life is like for the ordinary African living in the UK to an African living in the UK, particularly if it involved detached judgements about the challenges faced, etc.
I feel a bit weird saying this because I am in no position to make assumptions about how white people whitesplain