"Most appeals to the Court of Appeal require permission to bring the appeal. The lower court may grant permission, but this is unusual - it is a way of saying that the judge accepts the decision may not be right. More often, permission is refused and one has to apply for permission from the Court of Appeal itself. It is vital to initiate this process quickly. One has to lodge an "Appellant's Notice" within three weeks of the decision to be appealed. This is relatively easy to do, as the information required is not great. Nevertheless, full documentation and a skeleton argument explaining why the appeal is sound, have to be provided within two weeks after that.
In cases where permission for judicial review is refused by the High Court, the Appellant's Notice must be lodged within one week " Source
So, basically, the lower court, in this case the High Court, can either
a) allow appeal
or
b) refuse appeal.
If they allow the appeal, then the case will be heard by the court of appeal. If they refuse permission to appeal, then they are saying that they think they are completely correct, and therefore the party (in this case Met. Police) have to seek permission directly from the court of appeal.
It goes on to say:
"A decision is made on paper by a Lord Justice of Appeal. If the decision is to refuse permission, one can renew the application to an oral hearing. That will usually be before one or two Lord Justices. The other party or parties are not normally invited, but in practice they will often turn up to the hearing. They may still not be allowed to make representations - the point is that the prospective appellant must show that the proposed appeal stands a realistic prospect of success. If permission to appeal is refused at that stage, that is the end of the matter. One cannot take it further to the House of Lords (on the basis that you have been refused twice - in the High Court and Court of Appeal). (In this sense the Court of Appeal is, therefore, a court of last instance and as a matter of law must refer points of European law that are not acte clair to the European Court of Justice. In practice in such cases the Lord Justice(s) hearing the application for permission will usually adjourn the matter to a full three person court for further consideration.) "