Hi ad-uk, welcome to the forum.
There are a number of choices....
Colo - No Support
Colo - With Support
Dedicated
You can get "colo - no support" from Redbus, but you will have to use someone else for the other two.
More and more people seem to be going with managed colo (colo with support), as they save on the server rental in the long run and they own the kit - but can still get assisstance when needed.
I can certainly recommend Redbus (sorry Andrew - ad-uk did ask), although you should certainly shop around for bandwidth suppliers if you do decide to go for direct colo with Redbus - you would probably be going in to the Meridian Gate data centre and there are a number of good bandwidth suppliers to choose from - we use Easynet and Claranet (although we want another provider aswell) and we find them to be superb.
What you are going to find if you do just colo in Redbus is that you really will be on your own with regards to server setup / support etc, so it depends upon your technical capabilities and staff levels (assuming you want it monitored 24/7?). Redbus do 1/4 racks, although be careful to buy a server that fits (i don't think Dells 1Us do) as the 1/4 racks are specially modified (ie smaller depth wise than normal racks).
Their support guys are always on the end of the phone when you need them, but please realise that their support extends to reboots, unplugging cables and not too much more (they call it “Intelligent Hands”) - they certainly won't try to help you with a software issue (at least i don't think they will - i have never asked or needed to). You will also probably need to buy a hub or switch plus some cables - although in Redbus you don't need to buy a router as you usually get a 100Base-T ethernet connection from your provider - not throttled

.
All the Best