When I left the Uk I filled in the P85 form to say I was leaving the UK, my UK employer then paid me gross which was then subject to Swiss tax. I too went back and forth, but as some have mentioned you can only spend 90 days in the UK. When I did it they did not count arrival and departure days so a three day trip (arriving Tuesday morning and leaving Thursday pm) was one day. I think this now maybe tighter. Swiss tax is lower than UK (well it was for me).