Finding and applying for a Masters degree is different than applying for an undergraduate degree. You can search for them and apply via UCAS, but you can also apply directly to most Universities. You may need to write a personal statement as part of this application process and you may be invited to interview.
Entry requirements vary from institution to institution. It is common to need at least a 2:1 undergraduate degree, though there are some universities that will accept less than this, and some which will only accept people with a 1st class degree.
It is rare, but some institutions will accept people on to a Masters degree without any prior degree study. However, this is exceptionally rare, and only when you can demonstrate equivalent experience and knowledge from industrial experience. There are both taught and research Masters programmes available, so you will need to consider what you want from your Masters degree.