You will struggle to get a complete POE kit for £300. If that's the limit, I would start off without an NVR and add one later if needed and if you want to spend the extra cash. Our esteemed host could sell you a couple of decent Hikvision
DS-2CD2343G0-IU cameras at around £130 each and you would spend the balance on two 128GB SD cards, a POE switch and ethernet cable. Software comes free from Hikvision.
The amount of continuous recording you can get onto a 128GB SD card depends on the settings you choose for your camera and video. These are 4MP cameras capable of recording video at 2688×1520 pixels at 30fps but you wouldn't get many hours of that on a 128GB card. But by a combination of settings you could store a week or more of video before the card starts to overwrite the data.
The key settings to limit are:
Frame rate (hardly anyone records continuously at 25 or 30 fps). Try 15 or 8 fps.
Resolution - if you can manage with a smaller video display, reduce it to 1080p or 720p.
Bitrate - reducing the bitrate allocated to your video will reduce its file size and image quality.
Codec - try switching from the standard H264 to H265 video codec to reduce file sizes dramatically.
When you find that nothing at all happens in 95% of your continuously recorded video, consider switching to motion event recording only which makes your footage much easier to review and requires a fraction of the storage space. You could then increase the above settings and could even get a whole month of recordings onto the SD card.