I’d be looking at a basic Ajax alarm system rather than cameras necessarily. £275+VAT and they’re pretty much stick-on DIY devices and they’re really good at the alerting thing.
Had never heard of Ajax in my life, looking at the stuff that looks almost perfect! Really appreciate that, as it's likely saved me no end of money

(if, for some reason you would ever find yourself in need of supplies for Dungeons & Dragons, let me know, that's all I can help with haha).
Looking at that stuff then, how does this sound.
1x MotionCam Outdoor - Outside the back, overlooking the rear fire exit door we've got. I'd want the MotionCam over just a standard one so I can tell if it just someone taking a short cut, or a couple of Ne'er-do-well with a power tool and crowbars. I'll be there late most nights, so there really is zero reason for anyone to be back there after I lock up.
2x MotionProtect - Two situated in different corners over looking the entire ground floor. Might get away with just 1 as the range says its 12m and the ground floor is only 10m2 roughly. Don't think I need one upstairs, as you can't get access to that without using the stairs on the ground floor.
2x DoorProtect - One on the front door, another on the fire door.
1x Internal Siren - Hopefully I can set it up as a "warning" siren incase someone bangs the door or something accidently and not trigger the 113 dB of death the street one will give out.
1x Street Siren - Across the road is an entire block of flats that each have their living rooms overlooking my store. Also just to be on show and look the part.
Then the hub, a keypad and a couple of passes.
To top this off, thinking then of just getting a couple of generic TPlink style cameras that I can just use to monitor the store inside to deter stealing.