A lot of PAYG accounts have options to buy a "bundle" of min/txts/data from your account credit. These generally offer better value than the PAYG rates *IF* you actually use a majority of those mins/txts/data. These bundles expire.
I have an Asda Mobile account as a back up internet connection. Has a quid or two sitting in it and provided I make a chargeable call/sms to keep the account alive every few months it just sits there. If it's needed, I top it up and buy x GB of data. The hard part is remembering to make that chargeable call/sms, which reminds me...