Obviously factors like data caps, contract lengths and price are the key factors you should focus on, but each network has a number of interesting special features to make them stand out.
Unique features:
Voxi, for example, won't use up any of your data while you use social media apps like Facebook, Whatsapp, Twitter, Instagram and more. EE offers free subscriptions to popular services, Vodafone and O2 have their reward schemes - VeryMe and O2 Priorities - and most other networks have similar options.
Cashback:
A lot of SIM only deals will also come packed with cashback, giving you money back with your plan. This can significantly reduce your overall costs. This can come in the form of automatic cashback or cashback by redemption.
While cashback by redemption is the option that will save you more money, it is more complicated to deal with. Automatic cashback is a lot simpler, giving you all of your money back in one easy payment.
Gifts:
SIM plans can rarely come with a free gift. These could be smart tech like speakers, TVs or tablets. We've seen everything from headphones to smartwatches.
Of course, these aren't all that common and you need to be wary of a SIM plan that is offering a gift...but has included the price of the gift in your monthly bills.