@tds09k
The £5 minimum top-up is simply the minimum top-up that Vodafone can accept - nothing more sinister than that. Top-ups are multiples of £5 (possibly larger steps if you go for bigger top-ups), simply to keep the choices manageable. Otherwise drop-down menus etc would get pretty unwieldy.
So you put on £5 to begin with, and that will have 20p deducted per minute of call, or per standard text, or per 5MB of data, until you reach the daily £1, after which you can go on making calls, sending texts, or (upto a total of 500MB) using data without further charge for the rest of that day.
If it really is only for emergency use, your £5 should last a good long time. But something to be a bit cautious of is that you do have to use the phone occasionally to keep the SIM active. The Ts&Cs for Vodafone PAYG generally say that you need to make chargeable use at least once every 180 days; there's also a 90-day safety margin after that, and then the SIM gets disconnected, and you forfeit your balance. All the different networks have some condition along those lines, because Ofcom requires them to ensure that numbers which are totally unused get returned to "available stock" eventually.
So make a diary note to make a short call or sent a standard text every 4 or 5 months, and your number will remain active. But don't leave the phone in the glove box "just in case" - if your emergency is a year down the line, well, the battery may be flat, and the number will have lapsed. I may sound a bit flip, saying that, but people have come unstuck....
