You can also visit the National Library of Scotland Map Room in Edinburgh - they have a mindboggling 1.5 million sheet maps. https://www.nls.uk/collections/maps

While they don't sell originals, they do reproductions on demand for a lot less than 500k. I was just looking at a 1934 sheet for my wall, £27. Tho mainly I go to their site to jump back and forth in time, they have a great georeferenced map browser that lets you see how places have changed.