原句有误。they 跟句首的主语 they 是一回事，不是 money，所以用 are used 为错误。改为：

They will now have to keep London and its visitors secure with less money than they used to.

【翻译】他们现在不得不花比以前要少的钱，来保护伦敦及其游客的安全。

含义是：现在财政紧张，资金短缺。（可能是新冠疫情的原因吧）

比较从句是不需要补全的，如果单纯从理解角度，可以这样：

They will now have to keep London and its visitors secure with less money than they used to keep London and its visitors secure with.

【注意】比较对应项（much）及其被修饰的名词（money），在比较从句中决不能出现。即：much money。

比较项不能在主从句出现两次！“参与比较的名词” 如果在主、从句中作主语，会是怎样呢？例如：

More people hope to live in the countryside than used to.

More people hope to live in the countryside than did before.

More people hope to live in the countryside than before.

以上3句都正确。但在从句中，不能重复使用 people 或用代词 they 作主语。不能说：

More people hope to live in the countryside than they used to.

More people hope to live in the countryside than they did before.

总之，一句话：“比较项及其被修饰的名词”，不能同时出现在主、从句中。