Maintaining the Suspension of Disbelief

In fiction writing, it is the primary intention of the writer to attain and maintain the suspension of disbelief. The further away from reality the manuscript goes, the more doubt enters the reader’s mind. Apparently, if the genre is Fantasy then a new world may be built, and the activities within that world must be consistent. However, if someone is on an alien planet, with different animals and life forms but has a chicken sandwich for lunch, then there arises an element of disbelief.
In the genre Crime Fiction set in the UK in present times, the greater I leave reality, the more the scepticism becomes a problem. Everything must be a realistic action within the format of life in the UK, and then the suspension of disbelief will continue without a problem. The fiction I write is just a scenario that has not occurred, but is feasible to occur. The actions people make have not occurred, but would be likely in that particular case.