The inability to fly within Europe without an EASA license or conversion to same might not be of much consequence as the Johnson administration is unlikely to negotiate a post-Brexit-deal that will allow UK citizens to work in the EU. That wouldn't be of any consequence either if the Brexiteers' land of milk and honey projections come true and British airlines prosper in line with the rest of the economy even without a deal. I would also severely doubt the likelihood of a conservative government backing safety-based FTLs rather than going for lax rules as a means to improve competitiveness. Time will tell and empty pockets might thwart the best of intentions.