During the full 52 weeks of adoption leave, your employment terms are protected. You are entitled to all your normal employment rights except remuneration (i.e. wages/salary). Your contractual right to wages is replaced by the statutory right to Statutory Adoption Pay, or any enhanced contractual entitlement, if better. Check the policies in place where you work.
You are entitled to the benefit of any pay rises and improvements that are made to terms and conditions during the leave period.
You also have special protection if your role becomes redundant while you are on adoption leave. If you are made redundant when on leave, you have a right to be offered a suitable available vacancy if there is one. Your employer is not allowed to make you compete for this vacancy with other employees at risk of redundancy who are not on adoption, maternity or shared parental leave.
As regards pensions, any employer occupational pension contributions must continue to be paid in full throughout the period when your adoption leave is paid, but not during any periods of unpaid leave. (Statutory adoption leave is paid for the first 39 weeks of leave. The remaining weeks are unpaid. However, you may have rights under a contractual scheme that is more generous).
If you decide to finish your adoption leave and pay early and opt to take shared parental leave and pay instead, the terms and conditions of your employment will still be protected.