Herman30,
Since the UK is rather different from Finland regarding public fire places and relationship with nature, the whole scenario you describe would be very unusual here. That said, I think you might have been okay. Might.
I will elaborate on the scenario we were given at the Moot when we talked knife laws.
Two people going to the Moot (Merthyr Mawr, near Bridgend, Wales). One driving their car, one travelling by train. One coming from Birmingham, the other from London. They agree to meet at the Bristol train station, then drive together the rest of the way. Both have packed knives in their rucksacks. The train traveller has "good reason" to have a knife in their pack on public transport. They arrive at the Bristol station and have to wait for the driver to arrive. They are hungry and go out of the station to get something in a nearby café. They still have a "good reason", stopping for food on the journey is a reasonable action and they don't have anything else that they could do with their pack.
The driver arrives, parks their car and goes to find their friend in the café. They are worried about leaving their knife in their car since they know that the inside of a car is a public place, so they take their whole pack with them. They get to the café and get something to eat too.
The police officer providing this example said that the train traveller clearly had good reason, but the car driver, no so much. That while they might argue that they acted with best intent and good faith, they could have left their pack, and knife, in their locked car. Therefore they didn't need to have it with them in the café, where the knife was more accessible to them or anyone who stole the bag. That line of reasoning changes a bit if the vehicle has no out-of-sight storage, is a convertible, or a motor cycle.