It sounds like you are on the right track with your idea to calculate the annual average value for the bank rate. In terms of CLMR assumptions and Gauss Markov assumptions, as long as you are accurately and consistently calculating the average value for each year, you should not be breaking any rules. It's important to note, however, that using historical exchange rates from 1991 may not necessarily be a perfect match for the bank rates from 1980-2021. It's always best to try and use data that is as closely related and relevant to your analysis as possible.
Best of luck with your assignment!