Your CPP contributions will be deducted from your salary by your employer.
Your employer's contributions will be paid direct to the government. You will not see them, they will not show up on your payslip, and will have no effect on your final salary or tax situation.
This HMRC page explains relief at source. The payment was taken from your pay after tax, not before tax. So the statement about not including it in self-assessment doesn't apply.
This page explains how to claim back the repayment. If you do fill in a tax return (i.e. use self-assessment) then you can and should put it there. If you don't fill in a tax ...
"If you are contributing to an employer pension scheme which is a
final-salary or occupational money purchase scheme, you do not have to
worry about claiming the rest of the relief. Your pension pot will
automatically receive tax relief of either 40% or 50%."
but doesn't have any reference for that.
More usefully, Which? ...
Since you say you will probably emigrate in the future, I would advice you to stay away from anywhere your money gets locked in for any significant duration. Having your money locked in will do nothing but reduce your options. Find ways to invest that don't result in your money being locked in.
Real estate can provide reasonable (but not stellar) returns ...