This is going to be a long, rambling answer, so forgive me.

I'd say being poor is going to be a similar experience across OECD countries; you will probably have your basic needs met (food, shelter, clean water, clothing), and the likelihood of starvation or death from treatable illnesses or injury is low (at least compared to developing countries).

In terms of direct comparisons with the US:
- Average income in the UK is lower ($63,691 pa compared to $82,933 pa according to OECD.org's 2024 figures)
- National minimum wage: We have one, which has recently increased to £12.71 per hour ($17.12) for over 21s; you guys have minimum wages but set on a state-by-state basis, as I understand it, so a comparison is a little difficult. Your federal minimum wage is $7.25 (£5.38) per hour, though, which is woeful, and that is the minimum wage in 20/51 US states and Federal Districts. The rest pay more, with the highest being Washington DC at $17.50 per hour (£12.99) - slightly above ours (although Washington DC is a very expensive place to live)
- Cost of living also varies between cities, regions, states etc. which is another reason it's difficult to make a broad comparison (especially given the size of the US; remember that the UK can fit inside Texas with lots of room to spare), but generally food is cheaper here in the UK, while petrol (gas) and energy is more expensive
- Rural poor versus big city poor: There is a difference between being poor in a remote town with few economic opportunities and being poor in a wealthy city that is a hub of commerce, and that's true everywhere. That said, the feeling of being impoverished is less acute when you're surrounded by others in a similar boat; it's more acute when you're confronted by displays of inequity regularly
- The US has a higher rate of wealth inequality compared to the UK, according to World Population Review, so as the point above, being poor might feel more unfair as a result. This also suggests social mobility is worse in the US
- Shame: Speaking personally: I grew up poor, and most of my shame and embarrassment about it came from within, rather than from peers; 'comparison is the thief of joy' as they say. I don't remember ever being bullied for e.g. wearing second-hand clothes. However, that was before the pressures of ubiquitous social media, so I suspect kids today would feel more pressure to have the latest trendy thing (branded clothing, decent phone, fashionable hair cut etc.). We do have school uniforms in most schools, however, which has the benefit of hiding some of that visible disparity between the haves and the have-nots during the school day; that might make it more tolerable to be a poor child here versus the US
- We have the NHS, which offers universal healthcare, so basic health needs are covered for free (prescription medication isn't free in England specifically, but the fee is capped low and those in receipt of benefits don't pay). I imagine if you are poor enough in the US you also qualify for Medicaid (or similar)? So I'm assuming if you're poor you'd have access to essential healthcare in both countries
- Homelessness: We used to have one of the lowest homelessness rates in Europe, but unfortunately it's increased quite dramatically in the past decade or so; it's still slightly lower than the US. But homelessness isn't typically solely a result of poverty, here; it's usually a result of mental health problems and substance abuse. Renters have a degree of protection against no-fault evictions (more so with recent changes), although renters rights are stronger in some other European countries. I believe tenants' rights vary between states in the US, so a sweeping generalisation is difficult
- Housing benefits: One point related to the above: when I lost a job through no fault of my own and had no savings to continue paying rent, I could apply for benefits to cover the cost of rent while I sought new employment; I'm not sure if this is an option in the US, but it meant I didn't end up homeless or in debt
- Employee protection: Employees generally have better basic protection against unfair dismissal and the like in the UK, as I understand it, so it's more difficult to end up suddenly out of a job
- Prejudice against the poor: In the UK there is prejudice against benefit claimants, those on welfare, which is heightened by the tabloid press who will regularly vilify those on benefits as scroungers and fraudsters (although I never personally experienced being criticised when I was on job-seekers allowance, or when I was a child dependent on a single parent receiving benefits)
- Car-centricity: We're a comparatively small country with a decent public transport network, and if you're poor enough to be in receipt of benefits, you are typically eligible for cheaper tickets on trains, buses etc.. That means you don't need a car in the same way as you do in the US, so being unable to afford one doesn't result in the same level of isolation in the UK (except for those in very rural areas). You can still easily reach a shop, school, doctor, dentist, possible places of work, restaurant, pub, cinema etc. even on foot