The reason these Cycle Boxes exist is to make sure that people on bicycles are clearly visible to motorists and are able to traverse a junction in safety without being cut up or hit by motor vehicles.
Blocking a cycle box without good reason puts the safety of people on bicycles at risk.
Bicycles are the safest vehicles on the road.
Cars are the most dangerous vehicles on the road.
As a result of my complaint to the police, the driver was offered, and subsequently completed, a driver education course as an alternative to prosecution. They did this in their own time and at their own expense.
If you wish to report a road traffic incident in Leicestershire, here is the link https://www.leics.police.uk/ro/report/rti/rti-beta-2.1/report-a-road-traffic-incident/
Rule 178 of the highway code
Advanced stop lines. Some signal-controlled junctions have advanced stop lines to allow cyclists to be positioned ahead of other traffic. Motorists, including motorcyclists, MUST stop at the first white line reached if the lights are amber or red and should avoid blocking the way or encroaching on the marked area at other times, e.g. if the junction ahead is blocked. If your vehicle has proceeded over the first white line at the time that the signal goes red, you should stop as soon as possible and MUST stop at the second white line. Allow cyclists, including any moving or waiting alongside you, enough time and space to move off when the green signal shows.
Drivers of large vehicles should stop sufficiently far behind the first white line so that they can see the whole area where cyclists may be waiting, allowing for any blind spot in front of the vehicle.
Laws RTA 1988 sect 36 & TSRGD Schedule 14 part 1