It's not a defeat?

A victory? Hardly. An honorable draw? Stretching it. The BBC website records the view of the Prime Minister:

The withdrawal of British troops from the southern Iraqi city of Basra is not a defeat, Gordon Brown has insisted.
The 550 soldiers have handed Basra Palace over to Iraqi control and joined 5,000 troops at the UK's last base, near the airport, outside the city.
The Ministry of Defence said the handover of Basra province was now due in the autumn.
The prime minister said the withdrawal was "pre-planned and organised" and UK forces would take an "overwatch" role.

I rather think I incline to the assessment of Mr Cockburn in The Independent:

In terms of establishing an orderly government in Basra and a decent life for its people the British failure has been absolute.

Yes, I think that says it all.