Last one is definitly older than 3 month.

Name/Age: left to right: Ritsu (19), Mio (18), Yui (18), Azusa (17), Mugi (19)
Sex: all female
Description: See K-On anime, look it up if you want, or just look at the image and think about a generic moe anime.

Crime: Working without proper permit, violating terms of their visa.

Scenario: The Keions went on a trip to Britain (see K-On Movie), and played on stage in a nightclub by accident.

To deal with illegal immigration Britain has recently adopted a zero-tolerance poicy towards illegal immigartion. This law includes a mandatory death penalty for any alien working without a proper visa, as just deporting them has been proven ineffective. Playing on a stage in front of an audience definitly counts as "working" under this law, so the Keions are definitly guilty! The law doesn't explicitly state a minimum age, but for the sake of effeciveness it's probably supposed to also apply to minors. But you may or may not spare Azusa nevertheless.
The law also calls for a verdict to be reached within a weak after arrest and there are no appeals or requests for clemency allowed in such cases. The punishment shall be carried out immeditaly after the verdict.

Method of death and so on is up to the author.