AFAIK the underlying map data from Openstreetmap does not record Open Access Land in England so neither LoMaps not OAM maps can display it.
There is an Ordnance Survey type theme available for Locus that mimics the OS display of Open Access Land but the areas it marks are not actually open access as far as I can see.
Also, to clarify, land being open access does not make wild camping legal in England and Wales without the permission of the land owner. (You might be more likely to get away with it there however.)
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