I wonder if part of what happened here is a mismatch in service expectations. In many UK cafés, especially those we enjoy for their more relaxed approach, staff tend not to interrupt customers who are sitting quietly with a book. An open book plus one ordered coffee often reads as “I’m settled for a while and don’t want to be disturbed.” In that context, being left alone can actually be intended as polite.
That’s quite different from a more American-style service model, where staff are where staff are trained to check back frequently and actively upsell and move customers on. In the UK, unless it’s a full table-service restaurant, it’s often on the customer to signal clearly when they want something else. Eye contact, a small raised hand, or even popping up to the counter is completely normal and not considered rude.
I also think the wording you chose, while meant lightly, probably landed harder than intended. Even said with a smile, “I know I’m a woman of a certain age and therefore invisible” carries an implied accusation of ageism or sexism. That can put someone on the defensive very quickly, especially if they genuinely thought they were being considerate by leaving you undisturbed.
If the goal was simply to get attention and order more, something very neutral like “Excuse me, could I order some food as well?” would likely have worked without creating tension. If the goal was to gently nudge awareness, that’s understandable. But strangers rarely respond well to feeling corrected, especially in a work setting.
It may just have been crossed wires rather than anyone being inconsiderate. Sometimes the simplest, most direct approach is the most effective.