Yes I think you can.I was on a contract which had expired - so called up and
asked for new phone and took an online tariff £35PM 600 min and 1,000TXT
with an XDA Stellar.  Didnt like phone - sent it back and replaced it with
SE 890i which i also sent back.  Once returned I then called back and went
for simplicity @£25 (same mins etc but no phone) until I find a phone that I
do want.  Will keep this "rolling" until I'm ready.  They were fine with
this and even suggested it as I really didn't want any of the handsets
currently on the market.

Also - anyone know why they stopped doing Nokia N82?  In Dec it was a
"Coming soon" phone on the website (along with SE 890i) and late Jan it was
released BUT about a month later its no longer available by phone or online
BUT IS still available in O2 shops?????

have a look at this thread

http://www.esato.com/board/viewtopic.php?topic=169999

It answers your question and more