Aldi and Lidl offer every april or mai very cheap sleeping bags. They cost round about 15 £ and are OK for the money.
They are as warm as expensive bags, but last only round about 300 nights of use.
The packing volume is bigger than if you would buy an expensive sleeping bag, but that doesn't really matter in the beginning.
Such a cheap sleeping bag allowes you to invest in the other quality stuff I wrote about. And there it is more important to get satisfying good quality stuff.
The sleeping bag you can replace later.
I recommend to take a bag that is recommended for round about 0*C. If you wear your clothing in it that will even be warm enough until round about -3*C.
They usually offer an olive green version too. Take the mummy shape bag, not the bulky blanket shape bag!
Don't forget the option to sleep next to the fire under a lean to poncho shelter how you can see it in the video in the beginning of this thread.
That really makes a difference! Just store some spare fire wood next to your head and put it on the fire during the night!
If possible avoid pine trees and similar wood, because sparks could jump out of the fire and burn holes in your bivvy bag.
If your sleeping kit is out of the rucksack you should burn hard wood which holds an amber.
That's the old properly way to do it.
But attention about forest fires!
No fire in dry conditions!
If you read this thread later in the year and missed the Supermarket offers you can buy this sleeping bag here for example:
https://www.decathlon.co.uk/atlas-250-id_8539626.html
It's ok for the beginning too, especially during the summer. Or you have a look at the Snugpak homepage, they are a bit more expensive but offer a very good quality. There you get inexpensive long lasting sleeping bags.
Most of them are inside and outside olive green. That's perfect for stealth camping!