Your registration will take a short while to be recorded by the high level registrars (Nominet UK in the case of .co.uk) and until that happens the name will show as available. The fact that you've requested the name from WebMania isn't the point - they must record your registration with the domain registrars. Once that happens your name is yours - at least it is for two years.
Once the registration goes through the name will be 'tagged' to WebManias' name servers, and you can immediately publish a site to your WebMania server space. The site won't be visible on the internet until the Domain name servers around the world update their databases and record the location of your new name, so that browsers can be directed there. The update process can take 24 hours, although it's often faster.
Keep checking your account details on the WebMania site - you'll see your names appear once the registrations are confirmed.