If the .co.uk domain you want is still available, you can register it directly with the website builder. A .co.uk address (or another domain such as .com or .uk if you prefer ".co.uk address") is even included for the first year with MyWebsite Now. The package also includes a matching email address and an SSL certificate.
Use the IONOS Domain Checker to see if your .co.uk domain is still available. If your first choice is already taken, the tool shows you alternatives you can consider.