ViewRanger lets you buy by the grid plus I brought the whole of East Of England at 1:50k for about a tenner.

You buy credits and then download on a per tile basis. I thin 750 credits was about £15. I've downloaded tiles at 1:25K for 3 sets of routes near me (about 10 -20 tiles per route) and still have about 500 credits left.
 
outside is free, just make sure you have browsed the areas you want before your out of 3g/wifi range
 

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