Out if interest, why do you think they will send you anything beyond "we thank you for your interest"?In short, the points that I will make in the email are listed below. Please let me know about your thoughts, criticisms, suggestions and such. I will post the email transcript when I send it, along with any replies.
Introduction: greeting, "Please forward this to the relevant department or persons" and making my initial point
Benefits of a combined order: efficiency, cost-saving. Should I suggest the Class 378? If so, work for Litchurch Lane. A proven design, service around London
Conclusion
The orders will not be combined. Not in this universe. They may end up ordering the same units (though who will be ordering the GN ones is a huge unknown) but they will not join forces; it simply won't happen and some random emailing them to suggest doing so will change nothing.