My DB is here doing his 3 year MEng degree, he's just finished his second year and is doing really well (I'm so proud of him).
If you're determined to do it, it is possible and although things have changed since he applied, I'll throw my oar in
A 3 year degree in the UK is equivalent to a 4 year US degree. DB has looked at applying for masters in the US and has had no issues with degree equivalency, yes there can be an issue with 3 years degrees from other countries but not from a UK institution.
US Loans - My DB was originally going through IEFC and TERI, he's now with Sallie Mae, yes international fees are high but it's an investment in yourself and your future, my DB had terrible credit due to some issues with a credit card when he was 18, neither of his parents earned much, but his dad was able to act as a co-sponsor and he got his loans with no trouble, he's borrowed enough to cover fees and a large part of his living expensed.
Working - 20 hours a week is all you'll be able to work when doing a degree as a UK degree requires a lot of outside study and lecture time, this time requirement lifts during school holidays, so you can work during your summers and squirrel that money away. I supported myself on less than this when at uni, ahh those were the beans on toast days!
Support system - Personal preference. My DB loves the UK and has done fine, he doesn't know any americans in the UK at all but instead made friends with people at uni and work, they're a mix of every nationality. He just embraced a new life head on, and didn't look back, are you the sort of person who's open to that? Or will you break down because there's no good mexican food here?
Standardised testing - yes they don't count why would they? They're measured against a whole different set of criteria. This can work in your favour if you don't have great test scores, UK unis seem more open to mature students and students with other skills than just those with high test scores. Coming to the UK to study would mean a clean slate and because you specialise and just do the courses that are related to what you want, you don't spend time doing history of your state or whatever. This works if you know what you want to do, but doesn't if you're unsure about where you want to specialise. You'll have to think about how sure you are
All in all, my DB is due to graduate next year, he's near the top of his class (if not the top) and has a great job and friend set, I'm so incredibly proud of him, it can work, yes we'll have a lot of debt when he grads, but also he would have had an amazing 3year experience that he'd never have gotten, in the US he was fighting his GPA and taking classes and never getting anywhere, next summer he'll walk in his gown and I'm sure I'll be bawling my eyes out