"In one case, thought to be a legal first, a woman accused of perverting
the course of justice in a murder trial at Southwark Crown Court was
told that she must represent herself because there was no available
barrister after she had dismissed her previous lawyer."
She dismissed her lawyer, and I think it is quite common for defendants
to represent themselves after doing that. The alternative would be that
trials could be delayed indefinitely by dismissing lawyers, one after
the other.
It may depend whether the trial has started, or not, but once in
progress it's going to be difficult to get a significant adjournment for
a new counsel to catch up with what's been going on.