Uptime is the amount of time that a server has stayed up and running. This is usually listed as a percentage, like "99.9% uptime." Uptime is a great measure of how good a Web hosting
provider is at keeping their systems up and running. If a hosting provider has a high uptime percentage, then that means that their servers stay up and running and so any site you host with them should stay up and running too.
Downtime is the period or time when is a server is unavailable or offline. Downtime also referred as outage duration is the time period when the system fails to provide or achieve its primary function. During downtime websites that are hosted on the specific server will be unavailable and offline
As per my experience Webhosting UK provides web hosting services
with 99.9% Uptime Guarantee.