Thanks for the word of caution but I have to ask are you serious? This is a £35 polytunnel in windy North Yorkshire. Definitely well ventilated.
Yes I am deadly serious! I NEVER make fun of safety matters, especially when it might relate to combustion used in the living space.
There have been too many instances of caravanners and campers who have underestimated the effects of how well awnings and tents can actually seal against safe level of ventilation. Admittedly the problem is usually connected with also using an open flame heater (including charcoal BBQ's) of some sort. but you should also be aware that cabinet heaters (with a gas cylinder inside) and even catalytic heaters which claim to heat without a flame are all open flame appliances and will risk depleting oxygen from the livinare and replacing it with both Carbon Dioxide, and Carbon Monoxide (the invisible silent killer)
But as you have not defined what if any heating you might try to use I consider it sensible to advise you just in case.
I note that someone has suggests using a diesel heater. I have seen Youtube videos of some recent Chinese models which being sold as "mobile heaters" giving the impression you don't need a flue system, Any heater that burns or chemically reacts fuel with oxygen from the air will always need a raised level of ventilation or system separate combustion tract from the living space.