I have been issued a Swiss visa with one entry. I had requested for multiple entries. I will be entering Zurich on the first day of the visa. I also will be visiting the UK, Italy and France by train. Can I enter Switzerland again after visiting the UK, France and Italy?
No. You can visit France and Italy, which are also within the Schengen zone. There is generally no border control between those countries, and if there is a spot check, you will pass through with no problem.
The UK is not in Schengen, and when you leave the continent to go there, your single-entry visa will be canceled and you will be unable to get back in.
No. Since it is only a single entry visa, you cannot re-enter the Schengen region once you leave it. Going to the UK would mean leaving Schengen.