What is the difference between piracy and unjustified seizures? You don’t need to be a genius to know that Russia engages in unjustified seizures and Britain in legal ones.
In November 2018 Russia seized 3 Ukranian vessels which had entered its territorial waters in the Kerch strait. Nb; the ships entered Russian territorial waters even if Crimea is not considered; the Kerch strait is narrow, any ship passing through it will pass within 12 miles of the Russian mainland. The Russians claimed that the the ships had not cooperated with reasonable requests to cooperate with maritime authorities and said that they suspected them of being engaged in a sinister purpose.
In July 2019 the British seized an oil-tanker carrying Iranian crude (or fuel oil according to some accounts) in the waters off Gibraltar. Most press reports talk rather vaguely about it being “in Gibraltarian waters”. At any event, the strait of Gibraltar is less than 12 miles wide, and at some point in its passage, the ship must have been within EU territorial waters. The grounds are that the ship is breaking EU sanctions on shipping oil to Syria. Iran is not a signatory to these sanctions (not surprisingly) so presumably the legal claim is that the sanctions apply not just to EU firms but to any entity on or within EU territory – including a ship taking advantage of Freedom of Navigation rules to pass through the area.
The cases seem rather similar. In both cases the seizing power (Russia, the UK) claim that the seized ships were in their territorial waters and were breaking their domestic laws.
In the first case though the Western powers explain that the seizure is unlawful. This is the text of the G7 statement on the issue:
We, the G7 foreign ministers of Canada, France, Germany, Italy, Japan, the United Kingdom, the United States of America, and the High Representative of the European Union, express our utmost concern about Russia’s actions against Ukraine in the Kerch Strait and surrounding waters, which have dangerously raised tensions. There is no justification for Russia’s use of military force against Ukrainian ships and naval personnel.
We urge restraint, due respect for international law, and the prevention of any further escalation. We call on Russia to release the detained crew and vessels and refrain from impeding lawful passage through the Kerch Strait. 
In the latter case, of course, the action is presented as lawful:
We welcome this firm action to enforce EU sanctions against the Syrian regime and commend the Gibraltarian authorities involved in successfully carrying out this morning’s operation.
This sends a clear message that violation of the sanctions is unacceptable. 
Notice how British Prime Minister tries to mute the British role and tries to present this as some kind of action by the local authorities in Gibraltar.
Either there is “an international rules based order” or there isn’t. The most basic feature of the rule of law is that it applies to everyone without exception. Most rational people would see an international rules-based order mediated through the UN as 100% desirable. At the moment the Western powers negate such a prospect with their every step – while all the time claiming that they embody it – and everyone else is violating it.