Coming home from work tonight I put my bags on the seat next to me and read my book. A couple got on and the man said ostentatiously to his beloved: 'There's a seat. Sit there.' While I just moved my stuff onto my lap without acknowledging them she said: 'There are two seats together over there.' He insisted that she sat next to me while he gallantly stood. What a fucking tool.
You'd hate me then. I regularly ask people to shift their bags off EMPTY PRIORITY seats. I once asked someone to shift her jacket off an empty front seat too, yea there were sears at the back but I was in agony and I'm not going to out myself through more pain just so Joan from the front can give her jacket /bag a nice comfy seat.
Also pissed someone off on the train last year when out with friends, my back locked and I had to sit down there ans then. I stayed in that position for the sixty minute journey with the offended man glaring at me for daring to have the audacity before my friend had to help me out the seat and off the train.
Not really sure such instances can be compared with someone who wants to inconvenience others because they didn't book in time.several posters have given several scenarios as to why moving one seat isn't THAT big a deal, actually really is to some.
Just ask. But be prepared for people to tell you no. They don't have to tell you why.
An old adage of Mumsnet in regards and advice re assertiveness is "No, is a complete sentence".
Telling someone no you can't, is not rude. Expecting people to shift for you is.