I am thinking of getting a ticket to Switzerland from the UK just to buy a couple of electrical items. I don't want to take the flight; I want to go to duty free with my Switzerland boarding pass then just walk out through immigration again. Is this possible, or would I have to get a day return to take advantage of the duty free?
The longer answer:
There are two parts to this answer.
If you can actually do it depends on if you airport has devided Departure and Arrivals. For example in Zürich, Basel and Amsterdam it's possible to go through security and then follow the signs to the baggage claim. In Paris CDG this is completely different. The incoming passengers are very carefully routed through strange passageways and even over home made stairs to avoid any danger of meeting the others before they are dumped almost on the street.
If I remember correctly, the UK Airports international departures are completely seggregated from the arrivals.
The second part is the question if it's legal.
Technically you haven't left your country, so it's not legal to buy duty free goods and then go back out of the airport without declaring them and paying the proper VAT. Probably they won't catch you but i wouldn't try it for the marginal better prices you get at the airport.