I rather suspect the authorities do have rights to open baggage in UK airports without your being present. Even if its not covered by legislation, it might be contractual - ie a condition of using the airport etc.

I think the real difference (if there is one) is regarding the extent to which police they take up the rights they have and/or the quality and level of evidence needed before they do so.

A further check in Europe is the convention on human rights. Its articles don't specifically prohibit anything much - but state actions that could impinge upon them must be "proportionate" and "necessary in a democratic society" - else they can face challenge. So whatever the police do, it has to be justifiable to some degree.