@DHLawrence
Just a footnote, largely because I can't resist the temptation of corresponding with D H Lawrence!
If your sister goes into a Vodafone store, they will probably want to register the SIM to her, and it would probably be as well if she was ready-primed with a UK postal address, albeit a temporary one. The address of a hotel or guest-house where she is staying in the UK should be fine!
On a visit to Aus a couple of years back, I purchased a Telstra PAYG SIM from one of their stores, and they wanted a local address from me, but were quite content to take the address of the self-catering complex I was staying in.
Three other things to note:
1. In the UK, I believe it is only iPhones supplied by Apple which remain network-unlocked. Other iPhones lock to the network of the first SIM inserted into them. In case the same applies to iPhones in Australia, your sister should definitely do as @63johnw recommends, and make sure that the phone works with a SIM from a network other than Vodafone AU.
2. If your sister is moving on to other parts of Europe, and aiming to use the same SIM, she will be "roaming" - so will need to be aware of the Before You Leave section of http://www.vodafone.co.uk/explore/costs/travelling-abroad/ in terms of making sure that the phone isn't barred for roaming, and being adequately topped up to cover the costs of roaming (same page). It will depend very much on her likely levels of usage of data, UK minutes and texts what is the most suitable bundle, but a good point to start is here.
3. Calling home to Aus could get very expensive - see this page - unless she opts in to Vodafone International, which would reduce the cost of a call to a landline from £1.50 to 5p per minute. Unfortunately they've just made some 'helpful' changes to the page, as a result of which I at any rate find it very difficult to make head or tail of, but the basic trick is to call 36888 and opt in - for PAYG there isn't a monthly charge!
