Given the number of cases of soldiers being killed by "friendly fire" anyway it would have been a bit daft have a unit turn up in a place where they weren't supposed to be - and the British sector in the war was Basra not Baghdad. Just as it would have been if an Amrican unit had turned up in Basra to defend some building there.

Looking after security is most definitely part of the job of the people capturing a city, not of their allies who are tied up in another part of the country.

All part of the "rules of engagement."

Noone is criticising the ordinary soldiers for what happened. Some people at higher level made a terrible mistake. Very likely they would have been civilians.