Advance booking? I need advance warning

I went to the railway station at Ely in Cambridgeshire today. I wanted to book advance train tickets from London to Oxford in September. The middle-aged lady 'manning' the ticket desk wasn't there when I arrived and the other window was shut. Eventually she wandered over and didn't seem to understand that it was advance tickets I wanted.
"You never said," she said.
She didn't seem to understand that I only wanted the London to Oxford leg
"You never said," she said.
She looked up the tickets on the screen and decided that every seat on every train on the day in question was already booked and got cross when I quereid it.
"If there were no tickets available, the screen would be orange and say 'no quota'" she pronounced, triumphantly.
But I looked online this morning and there were plenty then.
"Well there aren't now. Perhaps you should have booked them online madam."
"There's no need to be so rude," I said, "you're supposed to be serving this community."
'Only difficult people end up doing community service, madam?' I thought it for her.