Boosting certain words/phrases in a should query?

Hi,
I am using the following query:

{
    "_source": [
        "title",
        "bench",
        "id_",
        "court",
        "date",
        "content"
    ],
    "size": 15,
    "from": 0,
    "query": {
        "bool": {
            "must": [
                {
                    "multi_match": {
                        "query": "london",
                        "fields": [
                            "title",
                            "content"
                        ]
                    }
                }
            ],
            "should": [
                {
                    "multi_match": {
                        "query": "London is a beautiful city and has a lot of amazing landmarks. I love the Thames!",
                        "fields": [
                            "title",
                            "content^2"
                        ],
                        "operator": "or"
                    }
                }
            ]
        }
    },
    "highlight": {
        "pre_tags": [
            "<tag1>"
        ],
        "post_tags": [
            "</tag1>"
        ],
        "fields": {
            "content": {}
        },
        "number_of_fragments": 5,
        "fragment_size": 300
    }
}

The rational of the query is that the word London must be present while those in the should query should just boost the score. What I would like to do is that within the should query, I would like to boost the phrase beautiful city and the word Thames. How do I do it?

PS: Content and Title are standard text fields with no analyzers applied on them.

Regards