The legislature thought of this
Jury Act 1977 s22 provides what happens if a juror dies or is dismissed before discharge (which normally happens under s55E "immediately after delivering their verdict".
The trial coninues if, under s53C, the court or coronor is satisfied that there will not be a substanial miscarriage of justice and:
in a criminal trial, there are at least 10 jurors remaining, or there are less than 10 and the Crown and each defendant agree, or there are at least 8 and the trial has been in progress for more than 2 months,
in a civil trial, if there are at least 3 remaining for a jury of 4 or 8 for a jury of 12,
in a coronial inquest, if there are at least 4 left.
If the jury is discharged under s53C(1)(a) either party may immediately appeal that decision in a criminal matter if they want the trial to continue (i.e. they think there won't be a substantial miscarriage of justice) and the proceedings are stayed (and the jury is not discharged) pending the result of the appeal.
It doesn't matter if the jury has completed their deliberations or not.