Britain has decided to replace the existing Trident submarine fleet. This link gives background, but the Commons debate is ongoing and no link is available that has full details.
Basically, we are building new submarines due 2024 to house our existing D5 missiles. The existing D5 missiles have a lifetime running to 2042 (in conjunction with US programs). At that time the PM stated “we will have an opportunity to partake in the development of the next generation of missiles in partnership with the United States” and noted “this will be confirmed in an exchange of letters between me and the President of the United States”.
My questions: Is this “exchange of letters” a common way for governments to reach agreement, and if so, when have they been used in the past? Can this exchange bind future Presidents?