O2's terms and conditions (
http://shop.o2.co.uk/tariffSmallPrint#Pay_Monthly) state:
# Inclusive messages on O2 Pay Monthly tariffs can be used for:
* texts sent in the UK to UK mobiles,
* texts sent from abroad (including Jersey, Guernsey and the Isle of Mann),
* picture or video messages sent from the UK and abroad (subject to having a compatible handset).
For inclusive messages sending one UK text uses one message. Except for customers who have an "Unlimited Bolt On", sending one text message from abroad uses four messages, and sending one picture or video message from the UK or abroad uses four messages.
Once you have used all your inclusive allowances, any messages sent to UK mobiles will be charged at 11.7p per message and any picture messages or video messages sent to UK mobiles will be charged at 24.5p per message. Any text messages sent whilst abroad will be charged at standard roaming text rates.
So looks to me like each MMS will use 4 txts from your allowance, unless you have unlimited in which case it seems you'll have unlimited free MMS's too (within fair use)