Well, I'm a British citizen (for now at least) and have been paying Pension contributions for yonks, but will also be eligible for
a Bulgarian pension when I retire. Now I do not understand what the problem is.
Prior to Britain joining the EEC there were plenty of British pensioners staying in Spain and drawing British pensions,
as they were also in Cyprus.
If I retire to Britain I should get my Bulgarian pension (probably get it paid into a Bulgarian bank and then transferred),
and if I choose to stay in Bulgaria there is no earthly reason why I should not get my British pension.
The only thing that I do know about is that if you are eligible for a British pension it will remain the same as when you start drawing it outwith Britain
rather than be index-linked.