Rewiring a boiler’s c wire for a smart thermostat
As it turns out, C is staring you in the face
As it turns out, the transformer is sitting right in the middle of your photos, and by matching the wiring diagrams from your boiler's installation manual to the wiring in your photo, we can determine that the C terminal on your transformer is the one with the white, black, and striped wires connected to it.  So, you can attach the spare blue wire from your brown thermostat cable to the C terminal on your transformer and use that as your C wire.