"Fast" ... what defines fast?
I can't even find a link to any files on their site to test their speed... (Maybe I wasn't looking hard enough!)...anyway luckily I know for a fact they run a CentOS mirror... so you can test out the download speeds here :
Index of /sites/ftp.centos.org/5.3/isos/x86_64
I did some tests for you from my connection
UKFAST :
00:38:37 (4.39 MB/s) - `CentOS-5.3-x86_64-bin-1of7.iso' saved
Goscomb :
00:45:23 (10.5 MB/s) - `CentOS-5.3-x86_64-bin-2of7.iso' saved
so they weren't the fastest there, but that could easily depend on peering - and other factors.
Now for website speed... that is dependent on many things :
Location of server dependent on visitor
You can tell when a site is located in the USA when you load it up, and you can actually measure the speed with firebug if you have it installed. For example load up
Dedicated Servers | Managed Dedicated Server Hosting (texas) - it took 20seconds for me acording to firebug! But to someone in america it would probably load in under 5. The Ukfast homepage took 2.9 seconds for me, and the Krystal homepage took 2.3 seconds.
Server Spec + Server Load
If you are using shared hosting, then the spec of the hardware and the contention of the hardware really matters. If you go with a budget host then they are going to be using SATA hard drives, 100mb network connections, and are probably overloading their servers. If you are willing to pay for a premium service then you will be getting better specifications, and a lower contention.
I noticed on their site that only their top linux server (
Linux server hosting with Red Hat Enterrpise, Fedora & more from UKFast) comes with SAS Drives, and the rest come with SATA drives and you aren't getting all that much RAM to service those machines. If you are running a website/s with databases then you need RAM and SAS drives if you want them to be
FAST!
Website programming/optimizing
This is probably the BIGGEST factor of all! If you want a fast website then you need to work hard to make sure you are doing as little computing as possible. You want to be serving files without hitting the database, and you want caching wherever possible. This obviously isn't needed for small websites, but once you start hitting the 10,000 visitors per day then you will start noticing the load on your machine from serving all these people with dynamic content!
I'm not trying to discredit UKFAST at all, but claiming you are the fastest in the UK is a sales tactic, rather than fact! If you fancy chatting technical about anything to do with fast servers/sites then give me a call

(hm my sig doesn't work yet! number is 0845 2000 699 - ask for Tom!)