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This is a strange one, I would be inclined to remove the shower head and see if when the shower is turned on and water flows out of the hose if that starts the boiler.
The boiler is activated based on a flow rate, so it may just be the flow sensor is starting to fail as the shower head is restricted the flow rate compared to a tap, and a new flow sensor would be reasonably straight forward to replace on a boiler.
If its not the flow sensor, I dont think it should be affected by other people in your building. I would start by running the hot tap in the bathroom and shower (with the head removed) and see if the water is the same temp under both tap and shower, if they are not it suggest you have a mixer \ temp regulator which is mixing cold water with the hot before it gets to your shower mixer.
I cant see any other explanation for your issue, and would look to start tracing the pipes between the sink and the shower to see what the cause may be.
A thermostat safety valve are designed to stop the temperature exceeding 40 degrees, they are normally located in bathrooms, to prevent scalding children in the bath etc.
The valve is often located under the shower tray or behind the bath panel so it can be serviced, most modern day mixer taps have built in thermostatic valves as well so it may be a failure on the mixer tap itself.
The valves look like the ones here and here