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These have the same stringent requirements as a passport, I'd either get one taken at a photo shop (quite a few will now take one for you with their camera set up for passport specifications and then print/cut it to the right size) or use a photo booth
Last year I had 6 photos produced and a copy on a USB drive done by the guy who runs a local camera shop for £7.
6 photos should be enough for quite a while and about the same value as the photo booth at £4.50 for the 4 as well as practically no chance of the passport office sending my application back.