The tactic is not actually legal. Almost every other European court that has examined the tactic has found that it cannot be justified - the police cannot legally detain hundreds or thousands of people indefinitely without probable cause. Only in the UK have the courts found that the ancient right to freedom from arrest without cause can be sacrificed in the name of "public safety." Thus, no legal standards apply - the police have and will kettle pregnant women, children, the sick, and the elderly for extended periods without access to basic necessities. No courts have shown a willingness to stop them, and your own people have become so unconcerned with their own liberties that there has been no public outcry.
You have reached a point in the UK where the police can do almost whatever they like, and even the Law Lords will not stop them. There is currently an application pending at the ECHR; perhaps your EU neighbors will save you from yourselves.
As long as it is supervised by an attorney, staff in a law firm can perform any number of legal tasks with the exceptions of providing direct legal counsel to non-attorneys outside of the law firm, and appearing in court to represent a client. For example, paralegals can prepare basic legal documents, provide procedural law information to attorneys for filing documents with various government agencies, and do legal research for attorneys.
Licensed attorneys can perform any legal task for which they have the requisite knowledge and training