If you suspect someone British has been arrested abroad, you should call the Foreign and Commonwealth Office (FCO) on 020 7008 1500, to make them aware of the situation. If you are outside the UK, call the British Embassy/Consulate in that country.

Staff at the British Embassy in the country of arrest will then try to make contact with the person to get more information, and will encourage them to sign up with Prisoners Abroad for additional support while they’re detained. 

You can see information about prisons in each country, as well as lists of local lawyers and translators, and contact details for British consular staff, on their website: https://www.gov.uk/help-if-you-are-arrested-abroad/y 

General information about how the FCO can assist can be found here: https://www.gov.uk/government/publications/arrest-or-detention/arrested-abroad-advice-for-british-nationals