I'm on a SIM-only tariff. When I ran out of data three days before my monthly refresh, I was appalled by EE's top-up pricing.
Whilst leaving them by text is tempting, it takes the fun out of ringing them up and saying "Your top-up rates with no roll over are just customer-hostile. Up my monthly data allowance for the same money or I'm leaving for GifGaf."
Just buy your phones outright, even if you need to use a credit card to do so. It gives you greater consumer protection against the phone vendor (Sales of Goods Act) and gives you greater leverage over the network operator.