Application for masters degree after completing a top-up is optional. You will be awarded a bachelors degree after completing the top-up. You can get a job with a UK bachelors degree.
Also, note that you will be eligible to apply for a Graduate Visa after your top-up degree has been awarded. The graduate visa will allow you to stay in the UK for two years to find a job after you graduate.
However, if you want to continue to study and do a masters degree after your top-up, you will need to apply for it.