If you enjoy being thrown into work with minimal oversight, then this is the job for you.
You'll get virtually no training (as a postperson) and be expected to keep up with people who've been doing the job for decades.
The management tactic is adversarial, and their singular objective is to save money, so they'll try and underpay you constantly, and not advise you correctly that you're entitled to things like mileage money (for going to other offices), travel time, and they'll try to imply that overtime is mandatory, which it evidently isn't.
You'll be on a twenty-hour contract, but it's expected that you'll go over this every day; it's 'unacceptable' that you work part-time, and frankly, the contract you'll be offered is not a reflection of the hours of a normal postperson.
Royal Mail haven't employed a full-time postperson for something like twelve years, so if you want a career -- a mortgage, a car, et al -- then this isn't the job for you. You may get full-time hours, but it won't be reflected in your contract.
It's a job reflective of the growing 'gig' economy, but while it isn't that bad, the job values flexibility (I don't know day-to-day where I'll be, what I'll be doing, and haven't been able to plan anything since I started), temporary positions, poor management attitude, bureaucratic culture of passing responsibility, and minimal job satisfaction.
If you want a job here, my advice is join the union immediately -- they're invaluable, and do not trust that the managers have your best interests (or even Royal Mail's) in mind.