An O2 SIM with Int'l Caller option has the cheapest rate for sms to foreign numbers, at 10 pence each. For other readers, it also has cheap calls, as does an Orange Call Abroad SIM for some countries.
http://www.o2.co.uk/mobilestariffs/tariffs...nalcallerbolton
There will also be costs for people sending you messages as well, of course, so it might be worth having 2 phones with NZ and UK SIMs, one for receive and one for send.
Several SIMs have bonus options of loads of inclusive messages to same or all UK networks when topped-up. Others sometimes have cheap rates to same network, like from 3 pence a go. This means you might even consider getting a couple of UK SIMs to give to family at home so they'd have send and receive phones as well.
If you will have internet access for part of the day, then some VoIP providers and other companies have cheap sms from 3 or 5 pence each - eg Skype, Babble, and the several Finarea brands Voipcheap, Voipbuster, etc.
There are also websites that can send free or cheap sms, though this needs careful attention to terms and conditions and other user reviews, as some of them may then start spamming with premium rate pay-to-receive messages. There are well over 100 listed on
http://www.textmefree.com/ - cbfsms seems popular, is free to UK numbers, and 5p int'l
In addition, some providers have sending possible via wap/gprs using a Java application in the phone. I've never got smsBug working myself, and Babble have a beta version which I haven't tried.
Most UK SIMs are very cheap eg 50p or ?1 on their websites, or actually free quite often, like 2 or 3 right at the moment, so you could order them to a UK address, then post a couple home if using my two phones suggestion for the most used contacts. Or I've got some here, or could apply for others to post on