I have just received an email on "choose your on-board meal in advance" so I suspect this is what the OP has received? It actually doesn't reference any specific booking and is just a general one promoting the service. My next flight where I could pre-order isn't until December.
Enjoy your choice of meal on board your upcoming flight
Dear KARFA,
We’re looking forward to welcoming you on board.
To make your journey as enjoyable as possible, why not choose your main meal from our on‑board menu? You can make your selection from 30 days up to 24 hours before your flight, at no extra charge.
This service is available in our premium economy class World Traveller Plus, our Club World business class and in First class on selected long haul flights departing from London Heathrow and Gatwick.
link - Choose your on-board meal now
The pre-order meal service is available on flights from:
London Heathrow to: Abu Dhabi, Accra, Amman, Atlanta, Austin, Bahrain, Baltimore, Bangalore, Bangkok, Beijing, Beirut, Boston, Buenos Aires, Cairo, Calgary, Cape Town, Chicago, Dallas, Delhi, Denver, Doha, Dubai, Hong Kong, Houston, Hyderabad, Jeddah, Johannesburg, Kuala Lumpur, Kuwait, Lagos, Las Vegas, Los Angeles, Madras, Mexico, Miami, Montreal, Mumbai, Muscat, Nairobi, Nassau, New Orleans, New York (JFK and Newark), Philadelphia, Phoenix, Rio De Janeiro, Riyadh, San Diego, San Francisco, San Jose (California), Santiago, Sao Paulo, Seattle, Seoul, Shanghai, Singapore, Tehran, Tel Aviv, Tokyo (Haneda and Narita), Toronto, Vancouver and Washington DC.
London Gatwick to: Antigua, Barbados, Bermuda, Cancun, Fort Lauderdale, Kingston, Lima, Male, Mauritius, New York (JFK), Oakland, Orlando, Punta Cana, Saint Lucia, San Jose (Costa Rica) and Tampa.
The link just takes you to an MMB log in screen so you enter the PNR & surname there.