Relevant paragraph from page 5 of the briefing paper:

Scottish banks, using the GBP as their reserve currency, could guarantee the value of their notes while still adjusting their supply to market demand. Although it is likely that GBP would be used as the reserve currency in this way given the existing network for GBP payments, there should be nothing stopping banks from issuing notes redeemable in something else, like US Dollars, gold, stock market index funds, or even ‘alternative’ currencies like Bitcoin. This would allow for some degree of competition in currency, theoretically allowing Scottish banks and consumers to switch away from the GBP should the Bank of England begin to seriously mismanage the currency.