Since the Magix website service doesn't currently offer the possibility to host CO.UK top-level domains, you basically have the following three options:

(1) Check whether you can set up some simple URL forwarding/redirection feature in your other hosting package. A lot of hosters will offer this option. You just need to put in the destination URL (i.e. the sub domain URL for your Magix website) and from that point on, typing in your domain name will automatically forward you to your website hosted on the Magix server.

(2) Website Maker has an "Export" function that produces a small piece of HTML code that you can integrate into another website to display your Magix website. So, you could basically put this into the "index.html" on your other hosting package and if someone visits that domain, you Magix website would be displayed. I would only use this approach if your domain hoster really doesn't offer the domain forwarding under #1, as that is so much easier and more straightforward to configure.

(3) Ditch your CO.UK domain and your entire other hosting contract and just obtain a new domain (e.g. COM, NET) in Website Maker. This might ultimately be a good idea as you would no longer have to pay for two hosting packages at the same time. Naturally, if your domain name is already well-established and already receiving a lot of traffic, you might want to hold on to it.

I hope this helps.