Want to break free from upfront fees and network subsidies? Welcome to the world of unlocked.

When it's time to buy a new iPhone, most of us turn to the four major UK network operators, or one of the many smaller networks that piggyback on them. But with the arrival of more affordable models like iPhone SE — as well as the continuing urge to upgrade to the latest and greatest — you may have been considering the alternative: simply buying an unlocked iPhone and popping in your current SIM card.

Unlike many other parts of the world, buying a new, unlocked smartphone isn't a new concept for British consumers. But if you're thinking of taking the plunge, there are a few factors worth considering: Here's a quick primer on buying an unlocked iPhone in the UK.

Why go unlocked?

First, a few obvious advantages. If you buy an unlocked phone outright, you'll trade a higher up-front cost for lower monthly fees from your network provider. The big players like EE, O2, Vodafone and Three have cheaper SIM-only plans that let you bring your own iPhone to their network as opposed to picking up a subsidized handset on contract. And so with a few exceptions, you're generally better off in the long run.

More freedom, more control.

Buying unlocked also lets you avoid getting roped into any credit agreements involved with taking out a contract; instead, you can take advantage of the many great Pay As You Go deals out there.

In addition, because you're not paying towards the cost of your iPhone across the life of your contract, you'll pay less each month. In many cases, you'll also be able to opt for a shorter contract: Many providers offer 12-month SIM-only deals. Some, including Three, even have rolling one-month contracts that let you cancel at any time.

UK providers use (for the most part) the same standards and cellular frequencies, and as such, compatible unlocked iPhones should work across any of the country's mobile networks — from the big four to smaller players like GiffGaff and Virgin Media.

The cheaper the model of iPhone you're buying, the more incentive you have to go unlocked.

An unlocked iPhone isn't tied to any particular network, and for that reason it often has greater resale value after you're done with it. That's alongside the freedom you'll gain from being able to switch providers without changing your handset.

And if you're traveling abroad, it's easy to pop in a local SIM, avoiding potentially expensive roaming charges.

Finally, with the arrival of iPhone SE as a less expensive option — from £359 unlocked — it might not be worth getting caught up in a multi-year contract when you can pay for the phone upfront and be done with it. The cheaper the model of iPhone you're buying, the more incentive you have to go unlocked.

iPhone 6s boxes

How to make sure you're buying unlocked (and not just SIM-free)

There's a really important distinction to understand before paying full retail price for an iPhone in the UK: Is it unlocked or just SIM-free?

Truly unlocked iPhones will work with any compatible network, and won't lock to the network of the first SIM that's inserted. Purchasing from an Apple Store — including Apple's online store — is the easiest way to guarantee that you're getting a truly unlocked iPhone with no additional fuss.

If that's not an option, all phones sold by Three, including iPhones, are unlocked — though you'll need to buy a Three SIM with the phone and activate it as usual.

In contrast, SIM-free iPhones, sold by retailers like Carphone Warehouse and Currys/PC World, aren't truly unlocked. Like an iPhone purchased on contract, these lock to the first network they're used with, and may not be usable with non-UK SIMs even after being unlocked. Basically, when you first buy and open your phone, it's unlocked to all UK networks, but once you start using it, it automatically locks to your operator.

From Three's unlocking FAQ:

"If you've recently bought or upgraded an iPhone from Three, it'll be unlocked as soon as you activate it over Wi-Fi or connect it to iTunes on your computer."

From Carphone Warehouse:

"iPhone is unlocked when you buy it SIM free but Apple's software means it locks to the first network you use it with."

That's why it's always worth double-checking where an iPhone was originally purchased if you're buying second-hand from the likes of eBay.

Are there any trade-offs? Things to watch out for?

Aside from the fact that you're paying more up front — and making sure you're really getting an unlocked device — there are a few potential caveats to be aware of.

Depending on the model of iPhone you want and the vagaries of your carrier's contract deals, there are times when it may be cheaper in the medium or long-term to pick up a subsidized phone (iPhone or otherwise) on contract, sell that phone, and purchase your own unlocked iPhone to use instead. For example, if a carrier wants to get rid of stock of a certain model, it might offer it with a tiny upfront price and a monthly fee not far off its SIM-only prices. In this case, if you're happy being locked into a contract for the standard 24 months, it might be worth taking the plunge.

It's also worth remembering that because of the way iPhone's software works, you won't need to worry about not having support for emerging technologies like VoLTE or Wi-Fi calling if you're using an unlocked iPhone. Most Android and Windows phones still require you to have a carrier-locked, carrier-branded version of your phone to use these features. With an unlocked iPhone — and up-to-date carrier settings — assuming your carrier has support for things like this, it'll just work.

UK readers, did you buy your iPhone unlocked, or are you using a carrier-locked model? Do you plan to change the way you buy next time around? Hit the comments and let us know