@Yeahyeahyeahnooooo you can definitely be warm! Some important things newer campers don't always know:
Sleeping bag temperature ratings are assuming certain conditions and they have 3 levels. If you get cold, make sure the "comfort" temp is in the zone you want. The ratings are assuming you are wearing long sleeve baselayers and long trousers and socks. Wearing a hat is also a good idea. Do not wear cotton as a first layer if you can help it - if you get sweaty initially, you will then be damp and then freeze the rest of the night. Merino wool or polypro are great.
The temp ratings are also assuming you are using a camping pad with an appropriate R-value (measure of insulation). Air mattresses are a BAD IDEA - you have to heat the air and the ground - you will never be warm. If you do use an air mattress, you need an insulating later between you and the mattress, like a thick synthetic sleeping bag. Better yet, use either a self inflating camping mattress or a thick closed cell foam camping pad. You do get camping air mattresses and these will have insulation inside or a reflective layer that serves the same purpose. If you buy a reputable brand (ie, Thermarest, Exped etc), it should not leak (our family owns at least 8 and no leaks in 30+ years) but bringing a patch kit is a good idea.
If you run cold, get a bag with a lower temp rating than the weather is forecast.
Some gear options assuming you are UK camping in the near-ish future:
https://www.cotswoldoutdoor.com/p/ayacucho-magura-5-c-sleeping-bag-G4HB3J0055.html?colour=1907
https://www.cotswoldoutdoor.com/p/mountain-equipment-helium-3-8-warmzone-sleeping-mat-D2424300.html?colour=98
https://www.cotswoldoutdoor.com/p/the-north-face-wawona-2-c-bed-sleeping-bag-G4HB3J0085.html?colour=157
https://www.ellis-brigham.com/vango-stellar-single-sleeping-bag-245097901
Both bags should get you down to 5 degrees C.
This one is a great example of misleading temp ratings:
www.ellis-brigham.com/the-north-face-stormbreak-20f-sleeping-bag-204693922
It's called 20F = -7C but that is actually the LIMIT temp. The limit temp = the temp when you will not get hypothermia but will be pretty miserable with a disrupted night's sleep. They don't quote the comfort temp for that bag, but it's typically 10-15 degrees warmer than the limit. Even the limit assumes appropriate clothing so if you want to sleep in less, you need a bag with a rating 10+ degrees cooler than the actual temp you're going to sleep in.
I work in this industry and honestly the biggest mistakes (and these are incredibly common, even among people who camp somewhat regularly) people make are:
- using an air mattress 2. wearing inappropriate clothing to bed and no hat/socks 3. not using a camping mat at all. Heating the ground takes a lot of energy!
If you have an appropriate temp bag, wear appropriate clothing and have a camping mat, you should be fine.