I agree with the reply from SFIM. However, the CAA has said that EU licences will remain valid for the UK for at least 2 years after Brexit, so perhaps check again with your employer.
The CAA has said that those who transfer can also have a UK FCL licence after Brexit.
The FAQ on their website says:
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If I transfer my licence to another EASA member state, can I be issued with a UK licence after March 29 2019
Yes, this process is under review. Further advice will be added to this microsite when available.
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The CAA's EU Microsite is quite well hidden.
You need to google "CAA Exit" to find the main page.
Then well down the page, buried in the text, you will find the word 'microsite' as a link, which takes you to the detailed stuff, including the FAQ
One wonders why it is so well hidden. Perhaps the CAA is concerned about losing control and revenue if the majority of pilots and organisations transfer to other member States.
There is a potential trap for you and any colleagues here. If you don't transfer you licence to another EU State and then in a few month's or years from now, your employer decides to become an EU airline (by moving their head office abroad), you will have to get a Part-FCL licence to continue to fly with them. According to current EU rules that will mean taking your exams again. And you would also need to do that if you wanted to leave your current job to fly for any EU operator. There is an article on this in the current issue of Flight Training News - and on their website. In your position I would draw that to the attention of your employer and ask them to think again.