Pre-masters is another option, but it might be more expensive I guess? You will first have to complete a year of pre-masters and then complete another year of masters which is 2 years in total, and the fee might be higher.
Whereas, a top-up is one year and only one year of tuition fees.
Applying through UCAS is usually required when you are applying for an undergraduate course, and sometimes a top-up degree. But usually, you can apply for a top-up degree directly to the university. UCAS is only advisable if you want to apply to multiple universities (up to 5), using one application. You can read our help article here about UCAS.