[source: link to gov.uk originally posted on page 1]At least 50% of the cycle’s use must be for ‘qualifying journeys’, i.e. commuting to work purposes;
50% of the journeys, or 50% of the mileage?
"Qualifying journeys": the clear assumption being that your commute journeys are consistent length which isn't necessarily the case.
My interpretation of that specific wording is that if you ride the bike for 1000 miles, 500 miles must be commuting (or going between offices or client meetings, or popping to the shop for more office stationary). Nowhere I've seen does it say you have to use the bike for 50% of your total commutes. Or 50% of your working days.
With that wording, you could drive the car all year, then just do a week of commutes on the bike. As long as your non-commuting mileage was less than that you'd be golden.
Worms, get back in your can!

It's basically a minor tax dodge that unlike a lot of much more egregious and lucrative tax dodges, has a societal benefit. So nobody cares.