I did some camping in a small tent in September. Technically a 2 man but it would have been a squeeze with anyone else. I stayed at campsites, although only basic ones, with toilets, hot water, and in most cases power available for charging devices.
Despite the uncharacteristic heat at the beginning of September, it still got pretty cold at night. Particularly on the clear nights, it was OK when overcast. My sleeping bag is more spec’d for compactness than cold, I took thermals to sleep in and glad I did.
Heavy dew in the mornings too. Was either packing up wet, or had to wait for the sun to dry things. Would think that would be a problem in a bivvy, how do you keep dry at night if you are touching the canvas?
A chap I worked with a long long time ago got into this a few years back and did some blog posts about it. His blog is here microadventure | Jason Webber Blog and you may find some inspiration or ideas from some of his posts.
I believe he doesn’t even bother with a tent - just a mat and a sleeping bag!
I’d just stick with the tent you have. That or use a bivi bag/tarp. As Buzz said, condensation is bad this time of year, so unless you use a gore tex hooped bivi you could get wet; plus you’ll have no admin room when it does rain however much £ you splash on one.
They’re definitely worth looking into. I turned an invite to a wedding in Ireland into an epic. Cycled to Heathrow, put the bike in a bag, just about got through security in my lycra, landed in Dublin, built the bike by the taxi rank and cycled 100km up the west coast of Ireland to get a shave and change into my groomsman’s suit. However I wasn’t convinced with wild camping and airbnb and hotels.com make a roof over your head cheap as chips.