Ok, answering my own question here. as I'm stuck under a breastfeeding baby and bored...
I've read the BBC article linked by @Ifailed and the report that it is based on (linked is in the BBC article). The report cites this paper bmjopen.bmj.com/content/10/1/e029688 as the basis for its claim that only 1% of packed lunches meat school meal nutritional standards. The most relevant information is on page 6 of the paper.
You packed lunch will meet the standard if it contains: starch, protein, dairy, fruit AND a vegetable. It must not contain any sweetened drinks (only water, milk and pure fruit juice allowed), confectionery (defined as anything with chocolate in it), or savoury snacks (such as crisps).
So, if your packed lunch meets those standards, you are in the 1%.
Not passing may judgement at all, as this is obviously only one way of defining healthy. But, as we are discussing it, based on comments in this thread: @CarlottaValdez would be in the 1% and @DinosApple might, depending on your definition of snack. @squeekums, @DipSwimSwoosh, @BoobsOnTheMoon, @Tinyhumansurvivalist wouldn't meet the standard.
Looking at Table 1 from the paper (hopefully attached here) white bread, ham, yoghurt, crisps, chocolate biscuit and a sweetened drink are all present in 25% or more of packed lunches. Which does sound pretty unhealthy to be honest.