Thank you for your question and patience, I’m Tom and I’ll try to help you.
“Permanent Residence” is the right for EEA nationals to remain in the UK without making any further immigration application, effectively that they can stay here without any time limit.
The stamp in your passport is probably the old style endorsement that EEA nationals used to get, the format of it has now been replaced but this does not mean that your stamp is ineffective.
This means that you will likely be regarded as having permanent residence, which means that you have the right to remain in the UK without making any further applications. Therefore, if the UK does ultimately exit the EU then your status is highly likely to be effected.
You will be fine, basically.
You can also apply for naturalisation to become a UK citizen if you wish and if you meet the criteria on the following page:
My goal is to provide you with a good service. If you feel you have received anything less, please reply back as I am happy to address follow-up issues specifically relating to your question.