The limits on a person being held in custody
without charge are stated here:
https://www.gov.uk/arrested-your-rights/how-long-you-can-be-held-in-custody
However, despite the reference to proposed legislation in your link, AOG, the Terrorism Act 2000 (as amended by later legislation, in particular the Counter-Terrorism Act 2008) already allows charges to be brought against people suspected of committing offences under UK law whilst abroad.
Once a suspect has been charged with any offence (from minor shoplifting through to multiple murder) the police (in the first instance) and the courts are obliged to grant bail unless there is a specific reason why bail shouldn't be granted (such as the seriousness of the alleged offence, the likelihood that that the person charged might abscond or the likelihood that they might commit offences while on bail).
It's likely that the people arrested (as per your Daily Mirror report) will have been charged and either refused bail or made subject to very rerstrictive bail conditions. (i.e. any proposed legislation is entirely irrelevant).