I am genuinely surprised you are having so much trouble with this. N Wilts planning policies that would appear to provide you the best opportunity are TM1 - General Policy for Tourism, BD6 - Reuse of Rural Buildings and BD7 - Farm Diversification. See the link at the end.
TM1 says "Proposals for new tourist accommodation will be expected to be located in, or adjacent to, existing towns and villages. Proposals for tourist development within the countryside that improves or extends the range of tourist facilities, including tourist accommodation will be permitted, provided that:
i) It would result in the conversion of a suitable rural building; or
ii) Forms part of a farm diversification scheme; or
iii) Is an extension to existing facilities, of a scale appropriate to its location.
Part of BD7 says
10.17 (Farm) diversification can include a wide range of enterprises of an appropriate scale, including non-food crops, woodland planting, recreation and leisure uses, the management of land to provide environmental benefits, farm shops, craft workshops and small-scale tourist attractions, contracting, equestrian businesses, woodland management, �pick your own�, nature trails, holiday accommodation and light industrial use. Such activities often help to sustain agricultural, forestry or horticultural holdings as viable units, and hence they can reduce the likelihood of farm fragmentation. However, PPS7 demonstrates that the appeal of the countryside is central to its economic prosperity and that development should therefore be of an appropriate scale and nature, so that they do not have an adverse impact upon the character, appearance or quality of the surrounding countryside. They should also be capable of being served by the existing local transport network.