123-reg.co.uk is a well known and well respected hosting provider in the UK, and is known more for its Domain Registration services than anything else being the largest domain provider in the UK. They currently have over 3 million domains registered, and host over 1.7 million websites.
About Dedicated Web Hosting
When thinking of dedicated server hosting the first thing that comes to mind is that you would have an entire dedicated hosting server to yourself. I.e. no-one shares any of the resources of the server with you like you would get with shared hosting and VPS hosting. Typically, when ordering a dedicated hosting service, you would have full control over the hardware, and you would have full control over the server, including the operating system and other configuration.
Usually, the dedicated hosting server is rented from a data center or third-party provider, but many data centers also offer co-location. Co-location is where you own the server outright, but rent space in a server cabinet along with other features and services, such as bandwidth, backups, support, etc.
You can read more about dedicated hosting here.