So I registered a domain and everything, and it's fine. the only problem is that only www.mydomain.co.uk (no, thats not the real url) works, it wont work if I just type mydomain.co.uk. I just get a 404 error.
Has www.mydomain.co.uk been left at its default value (ie mapping to the Force9 "homepages" server) or have you created a DNS entry for it pointing it elsewhere? I assume you want mydomain.co.uk to go to the same place?
Assuming you use a Windows operating system, what reply do you get if you enter nslookup www.mydomain.co.uk 18.104.22.168 at a command prompt? And if you enter nslookup mydomain.co.uk 22.214.171.124?
One way of doing this would be to enter mydomain.co.uk. (note the "dot" after "uk" -- very important!) as the "Left Field", select CNAME as the "type" and enter either "www" (with no dot following) or "www.mydomain.co.uk." (with the dot following) as the "Right Field". It may take a day or two for this to become effective.
This will make "mydomain.co.uk" an alias of "www.mydomain.co.uk", but if www.mydomain.co.uk points to the Force9 "homepages" server, it may still not work as a web address, because Force9's webserver will probably not recognise it should honour requests for "mydomain.co.uk". You may have to raise a Contact us request asking for it to be done. If "www.mydomain.co.uk" points to a host of your own, then once the DNS change takes effect, it will probably work (assuming your webserver isn't bothered about the hostname used in requests it receives).