I use Garmin’s Birdseye mapping to pull down OS maps. They can be zoomed in to 80m to the cm without blurring on my Etrex. Cost is kept down because you can buy so many square km of mapping and then download only the areas you are actually interested in, a bit like MemoryMap but instead of squares you can pull down rectangles. For example, when walking the SW Coastal Path I can download the edges of the coast as a series of rectangles but not the inland or sea areas that I am not going to use.