Most grammar books will tell you that you need “the” before a superlative like “highest” or “lowest”, but this is not always true. We use “the” when there is a noun after the adjective e.g. the highest(adjective) number(noun), the highest proportion.
When we put the noun before, we don’t need “the” e.g. the number(noun) was highest(adjective), the proportion was highest. So, compare these 2 sentences: The UK had the highest rate of unemployment. The unemployment rate was highest in the UK.