UK border control for US pport holders?

Hi I am going to the UK for 3 months soon with my German fiance and was wondering if I can come in on a visitor visa on arrival if my intentions are to stay with a friend for a month in a beautiful farm community in scotland.. I know I have read much about volunteering and farms and how you need a work visa for that but my intentions would be to pay money for the month I was there so we could be on retreat for some time of relaxation. I am just wondering if border control wont accept this? thank you