Many countries outside the UK refuse to grant bail before trial and often detain people in solitary confinement

Being caught by UK customs with a small amount of class C drugs, for personal use only, may just get you a financial penalty rather than a court case, however you will get a customs record, and if caught again you will get a criminal prosecution. If someone you are travelling with is caught with drugs, you are likely to be searched and questioned too. If you are caught with drugs on you as well, no matter how small the amount, you will both get the same legal penalties.
You will still get a criminal record in the UK if caught with drugs abroad, which may effect future employment. This may also mean that you are refused a visa for the some countries including the USA.
If you've been caught with drugs abroad, you're unlikely to ever be allowed to visit the country again.
If you get injured or ill as a result of drugs, your holiday insurance may be invalidated and your tour operator can refuse to fly you home.