even on "protected" NCB id be very skeptical about your insurance not going up for a non-fault accident.
almost ALL insurers for some fucking gay pathetic reason view any non fault accident as a higher risk.
Your premium may not have gone up, but it also may not have gone down as much compared to no accidents.
for most insurers youd pay well over the cars £700 value in higher premiums over the next 4/5 years.