In Endeavour S05E05, Bagshot and Morse discuss some spying/agents stuff between Britain and Russia during the Cold War (1960s):
Bagshot: There have been rumours for some time that HMG (Her Majesty's Government) has been penetrated to Cabinet level. Maybe even beyond.
Morse: No. I don't believe that.
Bagshot: You may not, but our friends on the Continent seriously think so. Why do you think de Gaulle is so determined to keep us out of the European Community? Buy British, get Boris.
Who is this "Boris" here and what does "Buy British, get Boris" mean in this context?