I don't really think it matters where a person's sympathies lie, they are allowed to protest without losing their job.
What might complicate this slightly is that he was in his workplace. So asking, if a driver was leading a protest about another political issue on the tube, would he be let go, seems a reasonable comparison to me
Agree with this. The issue isn't that he sympathises with the Palestinian people. The issue is using his workplace to protest about political issues.
That said, I think Dianne Abbott was very foolish to write that tweet.