As per a previous question answered this is optional. But I am wondering, what is the reason behind making the decision to remove the words 'European Union' from the cover of UK passports issued since March 29th 2019 (the original date set for the UK leaving the EU)?
It seems obvious they were preparing for Brexit for end of March.
The decision to remove the European Union label was made in the expectation that the UK would be leaving the EU at the end of last month, as scheduled.
But still owing to some practical (stock-depletion) considerations they even printed EU passports after that date:
The Home Office said some people may still receive the old version until stocks run out.
The mixture of burgundy passports (with or without EU printed on them) is also supposed to be temporary.
Dark blue passports resembling the pre-EU British design are due to be issued from the end of the year. [...]
A change in the design of the UK passport has proved a rallying point for Brexit supporters, with former UKIP leader Nigel Farage describing the 2017 decision to bring back the dark blue design as "Brexmas".
Answering the 'why' and not 'on what legal basis' ...
- The UK government had decided to implement the popular decision to leave the EU. There was a deadline in UK law.
- Once issued, passports are valid for several years. It is not very practical to recall and replace them.
So it seems logical to change the passport design early in the Brexit process, to minimize the time that UK citizens are moving around with an EU passport. An EU citizen without the words 'European Union' on the passport will cause fewer double takes than a non-EU citizen with the words 'European Union' on the passport.