You can use this letter if you specifically requested certain facilities or amenities for your holiday, but they weren't available when you got there. This was despite the fact that the travel agent confirmed that they would be available. The facilities and amenities can also include additional services arranged by that travel agent.
You can complain either to the individual travel agent directly or to the travel agency business and seek compensation for breach of contract and loss of enjoyment. This document is suitable for consumers (not businesses) in the UK.