Explanation: There’s a little bit I wrote back on The Old Place which people liked, so I’ll post it here as way of explanation for the ridiculous k/d ratio (Some Roman sources claim 250,000 Britons, but historians generally reject that as an exaggeration or, at minimum, including noncombatants)
Imagine: you’re a Briton. You know the Romans claim you have a quarter of a million people, but you know you ‘only’ have 100,000 in the army. Still, you outnumber these Roman dogs 10-1! You and your fellows are mostly tribesfolk, farmers with spears and swords and shields. A few dedicated old warrior-nobles are with you who have armor as well. The Romans do some weird formation thing to press up against a natural obstacle, and your Iceni leader gives a bomb-ass speech.
You and the army swarm down the plain. You have no training in formation movement, so you’re a crowd, a mob. You know that the sheer force of your numbers can overrun these pitiful 10,000 Romans. As you close the distance, shouting warcries, the Romans begin throwing their javelins. Men go down, screaming. Others have to discard their shields. You’re tripping over the debris and wounded as you move forward to engage. The weight of those behind you presses you ahead nonetheless, unless you yourself wish to be trampled. As the Romans exhaust their missiles, they begin a counter-charge.
With a ferocious cry, you strike your spear against the frontmost legionary you see. His shield is nearly the size of his body, and he deflects the blow. The momentum is too much now. The wave of Britons has no choice but to crash against the shields of the Roman legionaries. Your spears and swords are tangled in the mess, while the Romans, with their ugly little glorified knives, use short thrusts from behind their shields to carve up the unarmored men forced into close-combat.
You strike high and manage to get past a Roman’s shield! Success! Only, he’s wearing thick armor that ensures your blow did nothing more than break a chain link and bruise him. He’s pissed now, but more importantly than that, his buddy was watching for a dumb barbarian like you to strike. He plunges his sword into your side. You stagger backwards, managing to slip through the roiling wave of Britons, blood seeping from your wound. The Romans advance in formation, each one protecting the flank of the other, moving forward with brutal precision. Anytime a Briton is exposed, or turns the wrong way, or is jostled forward, he gets a gladius in his gut for his trouble.
Their large, heavy shields bash against the lines of the Britons, and disorders them further. Some fall - the Romans give them a quick stab and then stomp them with their hobnailed sandals. You wonder that the Romans can hold out for long against you - sheer exhaustion should whittle away their numbers. But the wily Romans shift around, and bring fresh men to the front, and the exhausted and wounded to their rear. These men are as a stone wall. It begins to feel hopeless.
Then the Roman cavalry slams into the flanks, thrusting and trampling with impunity. Everyone is trying to face everywhere at once, uncertain where the next Roman blade will come from, or if it will come from a legionary or cavalryman. Some try to run, but there are too many people, and no one trained to move in a formation, much less for an orderly withdrawal. Now men are being stabbed literally in the back. The Romans smell blood in the water. In the panic, your fellow Britons end up funneled by the wagons and families they brought with their camp. There’s nowhere to run, you don’t have room to swing your sword, and the Romans are still advancing with mechanical precision, guided by their officers and their battle standards.
Your only hope of survival is to push and squirm and trample whoever of your own you can, in the hopes of making it far enough back to flee. How many do you think escape?
The Battle of Watling Street was not well-organized tribes of warriors facing the Romans. It was an angry mob with a few warriors mixed in facing the most well-equipped and professional army of its day.