If your budget is only £40 and you want a 128GB card...have you checked to see just how much these cost? You won't have much change from the memory card to buy a camera, if any. And if you only want to spend £40 on a camera, chances are the footage will be of a lower quality anyway and you won't need 128GB because the files will be smaller. 128GB is usually for 1080p upwards and generally higher bitrates typically 20mbps or higher, or dual channel cameras recording two video streams to one card.
Pick a camera and check its bitrate online, then use DCT's own Recording Time Calculator to see how many hours you'll get from 32GB, 64GB and 128GB cards. Now ask yourself, how much recording time do I REALLY need?