AOG, Its a good question and I can see why some one would raise the issue of police bail given the circumstances of this case. It is difficult to comment on this particular case due to the limited available information.
In summary, when people suspected of crime are arrested they can only be detained and questioned for a certain amount of time. In some cases, investigators would collate most or all of the evidence before arrest to ensure they do not run out of time they have with the suspects in custody. In this particular case, the four accused would have had to be detained as soon as they were identified to secure and preserve any evidence and to protect the public etc. While the accused are detained the investigators would then have to go identify and correctly secure any evidence to put to the suspects. (CCTV, witness statements etc)
Dealing with four detained foreign juveniles for such a serious offence would be very time consuming and without sufficient resources the risk of running out of the allowed custody time limits would be high. (Also each of the accused would need an appropriate adult present, interpreters and solicitors). Other specialist investigators would also be needed and all these people have to be identified, appointed and briefed in the very limited time that the police are allowed to hold the accused for. After all the (or sufficient) evidence has been assessed and the accused interviewed then the facts have to be presented to the Crown Prosecution Service for decision and if there is little custody time limits left then unfortunately bail has to be considered.
If a person is formally charged with such an offence then yes investigators should be looking at refusing bail but it is not automatically granted and in fact often refused by the courts, even in some serious cases.