Since you refer to age 67, I assume you will reach that age post 2021 when the State Pension age goes to 67.
Answering you question is tricky as there is likely (not yet confirmed) to be changes introduced to the calculation basis in 2020. If the proposed changes go ahead, an A contributor will require 30years of A contributions to get the full State Pension. So 12.5 years might get you a proportionate Pension of c40% of the full State Pension.
Under the current “totals and averages” basis of calculation, I think you would get a roughly similar percentage.
It is expected that the Gov will produce a new Pensions Framework document in the near future which hopefully will clarify future strategy.