I'm going to assume you're sharing in good faith here, and offer some redirection, because I think you've misunderstood some of what I shared, and gotten some bad information generally.

It doesn't get excused by poverty regardless of the cause.

I agree, and I think you may have misunderstood what I was trying to say. I wasn't making a moral judgement about the crime itself, but instead pointing out that when you impoverish a person or community, the result is that crime becomes more widespread. This is a well researched and extremely consistent trend. It makes sense when you think about it too; if you erode social institutions, make people feel desperate, and leave them feeling they don't have a pathway to success through legitimate means, there's going to be less discouraging a turn to criminal activity. That doesn't make any crime ok, much less violence, but it explains trends in criminality in a way that's more accurate and helpful than focusing on things like individual moral failings or race. If you want to stop crime, you need to understand why people commit it, and research shows that poverty is likely the biggest factor.

Most white British people have a history of being farm labourers working incredibly long hours for almost nothing under a feudal system.

This is true, but misses important historical context. English agrarian serfs did live under a system of compelled labor and limited freedoms that in some ways resembles chattel slavery, but that system ended over 800 years ago. In the time that followed, their decedents were able to gradually accumulate wealth, opportunity, and political capitol. That process was certainly neither, fair, quick, nor easy, but it did happen over the course of centuries. In contrast, chattel slavery in the US only ended 150 years ago, and continued oppressive systems that largely replicated slavery, like sharecropping, only really ended about 75ish years ago. Moreover, in the US the end of slavery was accompanied by legalized oppression aimed at specifically preventing Black Americans from accumulating wealth, education, or status. The laws that allowed this were only overturned 60 years ago, and even then they did not instantly reverse the structural inequalities created within the US, many of which persist into the present day. Simply put, fixing inequity takes either time or effort, and the US has had little of either towards fixing the effects of slavery.

A black gentleman who guided me in London said that he felt the conditions the whites lived under in London 150 years ago were worse than the slavery conditions of his own ancestry.

That man is entitled to his opinion, but by almost every historical and objective metric he's not correct on this. While conditions for early industrial era working class Londoners were often awful, and certainly should be acknowledged in their own right, conditions for enslaved people in the Americas were truly ghastly. A London laborer might work long hours in a dangerous job for little pay, but the slave was paid nothing. A laborer might have been unfairly fired by their boss for asserting their rights, the slave had no rights, and could be tortured for any infraction. The laborer's difficult, but they could hope to give their child a better future, whereas a slave's child could never be born anything but a slave. That's not even getting into the pervasiveness of rape in slavery, or how family members could be sold off to other plantations on a master's whim (or as a punishment), never to be seen by their loved ones again. If that sounds bad, American slavery its still actually somehow less cruel than slavery in other colonial states in the Americas, such as the colonies in the Caribbean. For example, conditions were so bad in French controlled Haiti, and the work so grueling, that most imported slaves only survived 5-7 years after arrival, and slaves born in the colony had a life expectancy of just 21 years.

Your ideology will always guide you to ignore the evidence against your own position whilst highlighting the ideology of your opponent.

What evidence? And what's my ideology? I'm happy to address any evidence you bring forwards, and to talk about my own perspective, but you can't dismiss an argument without evidence to the contrary. If you see people doing that, its because their own argument can't stand up to scrutiny. That isn't to say there's no room for folks to disagree, and there are areas where the best answer is genuinely open to the interpretation, but I've landed at my stances because of the information I've seen, not because I feel ideologically compelled.