Take a look at
http://www.hmrc.gov.uk/cnr/nr_landlords.htm.
Basically, you fill out a (UK) form to apply to receive your UK rent income tax free (on the assumption that your "usual place of abode" is not the UK) and send this off to HMRC.
They reply, giving approval, and a reference number. You give this reference number to the letting agent, and they pay you the rent tax free.
Then, when you do your Swiss tax return one of the things you have to declare is the rent income received, and any costs incurred (letting agent fees, anything else you've had to pay out in that year). This is used as part of the calculation to figure out your Swiss tax liability.