Fair enough, but I wish I had 5 leva for every lawyer who has become rich by helping Brits!
Unfortunately you seem to have been given some misinformation. When the UK leaves the EU you will have to apply for a different visa, there shouldn't be any problem as you already have a 5 year residency visa.
Of course, the UK might not leave, in which case you are good for 5 years and then you need to apply for a ten year visa.
Driving licences are also an issue - British licences need to be exchanged for Bulgarian .
This link will give you access to the latest updated information from the UK Government on living in Bulgaria and Brexit -https://www.gov.uk/guidance/living-in-b ... nformation
You can also get the latest information on the Bulgarian Government website athttps://www.mvr.bg/en/about-the-ministr ... out-brexit
Hope this helps.