Generally they have mechanisms to detect tampering with the ECU, and hence the warranty is void. I don't know how they do it on the Up, perhaps some form of seal.
Any software change sets the td1 flag (Google it, plenty of info out there) which is detectable by VW when they hook your car up to their online diagnostic system. Even if you revert back to your original map the flag remains set.
You can't reset this flag with vcds, nor can a dealer technician with a dealer level tool.
If your car goes in for warranty work it will be scanned and the remap will be detected. Whether or not this gives VW grounds to reject any claim is debatable. A remap on a turbo'd car increases boost pressure, if your claim is for a turbo problem I suspect it will be rejected.
At the end of the day VW can afford legal action more than we probably can so if warranty is a concern don't remap.