UK equivalent of South Africa MixIt

My friends kids' in South Africa use MixIt (http://www.mxit.co.za/wap/)
whereby they send SMS messages at a fraction of the cost of a normal sms.
Is there an equivalent product like this available for us users in the UK?
I think it uses GPRS. Can anyone recommend a "bolt-on" GPRS tariff for O2
users that would allow this program to be used? (Tariff at present 400
mins/100 txts and I think 100kb data (not much!))
Thanks