My 2p worth.
A basic foam mat is just fine, don't be fooled into spending more than you need, the £5 Amazon job linked above by Mr Thrills, looks good.

"Don't let the sunshine fool you"
The basic function of a mat, foam/closed cell foam/inflatable/semi inflatable/pop up self inflator etc, is to insulate you from the ground, not to mimic a slumberdown extra comfort king sized waterbed. Camping is camping after all.
So, the main function of the mat being insulation, it's that function I'd be focused on more than anything else. So the first question would be where am I going to use it?
For UK lowland camping (a good place to start) a couple of large Cardboard Boxes opened out and placed under the sleeping bag makes a profound improvement over going sans mat. Cardboard isn't practical for carrying around, it gets damp it's bulky, and a tad trampy for the discerning aspirant gentleman bushcrafter
The next step up is that £5.27 mat linked by Mr Thrills, it maybe a simple low cost mat but that mat looks to me to be around a 5 or 6 LOC (layers of cardboard) on the cardboard box scale, so more than adequate for 3 season UK lowland camping, for 4 season camping, just buy two £5.27 mats
In cold weather the mat is more important than the bag, IMO. Heat loss to the ground is the biggest factor when it comes to staying warm, and staying warm is 90% of being comfortable.
The mat rolls up and attaches to the exterior of ones sack, whilst on the move, two rolled up together work in the same way one just ends up with a bigger bundle, mats rolled up for carry are best then stowed in a bin bag to keep it dry before tying off to the rucksack. 2 of those mats would trump almost all the others on the insulation stakes, for example.
Comfort? There's more to comfort and getting comfortable than fancy Carlos Fandango kit. As mentioned above we can get 90% of our comfort by getting and staying warm. The remaining 10% comes down to where we park ourselves for the evening.
When I'm planning a camping site, the first thing I do (after finding a likely spot) is to get my mat out and lie down in a few different positions and then decide where the best spot is, then I set up around that.
Tents offer more protection from the elements, and biting insects, if managed correctly. Tarps are more flexible but require more experience IMO to make suitable use of. Bivvi bags are good stand alone (if the weather isn't too rank) or used in combination with a tarp set up.
If one was handy with a tarp, had semi reliable tent or other rain proof shelter, bothy, howf etc, then one could easily forgo the bivvi bag (
***NB; in a situation where your life may depend on creating shelter, upland and or winter camping when the tent or tarp blows away, or you're otherwise exposed and starting to go hypothermic, then you should have a bivvi bag in your sack regardless, as an emergency backup, standard practice, plastic is good for an emergency and low cost, goretex; something you could work towards, most people dying outdoors in the UK die from exposure, and most would have survived if they'd just climbed into a plastic bivvi bag whilst they still had faculty enough to do so).
A good tip for getting to sleep for a 1st night in a tent when one is new to it, is to go to bed very late and rise very early, or just stay up all night; the day you plan to campout, ensuring that by the time you're all set up, had your scoff, allowed the fire to die down, got sick of whittling sticks etc, one is proper knackered.
After sleeping the sleep of the very sleepy for an evening, under a tarp/in a tent, on your mat in your sleeping bag; on the ground, that'll be you in the way of it.
For pillows etc, I just arrange the clothes I'm not wearing and use them.
£5.27 thats all I'd spend on a mat initially, it's all you'll need. It won't deflate puncture etc, as it is what it is and whilst it may seem bulky when compared to fancy pop up pocket self inflatables etc it doesn't really weigh anything worth worrying about and is attached to the exterior of the sack, so it doesn't take up valuable pack space.
For my own part, I've done years of camping, I've a £400 tent, a £100+ sleeping bag, a £200 bivvi bag, but I still park my backside on a simple foam mat and don't see myself going all inflatable any time soon. For me, my mats are by far the lowest cost elements of my camping sleep set up.