If I relocate to another country, will I still be able to access the Play Store and use my purchased Apps, Books, Magazines, Music, Movies and TV?
He will still have access to all his purchases.
He will NOT be able to access the UK Google Play Store, unless he uses alternative methods (VPN/Tunneling/Market enablers..).
Assuming he managed to access the UK Play Store (2), he will be able to purchase as normal provided that he has access to his UK credit/debit card.
In order to purchase a Nexus device, he has to use his UK credit/debit card, and select a UK address for shipping, and then have someone forward him the package.
Provided that the devices he purchased was activated in the UK, he should still the Books and Movies apps on his Android phones/tablets. Some devices however will automatically freeze these apps on boot if the device was not activated in a supported country. 


NOTE: Devices, Books, Magazines, Music and Movies and TV are NOT available in Switzerland without using methods that bypass the restrictions.

NOTE: The answer above is based on my experience with dealing with Google purchases, market restrictions, and international shipping. I have never dealt with the Swiss Google market specifically, but the experience should be the same.