path from uk student to US lawyer

if you just do a uk law degree can you still go to a really top firm?

You make it sound like someone can just waft in to Oxford or Cambridge, waft into an Ivy League Law School, and then waft into a large law firm. The routes which you mention are intensely competitive at every stage and only the most academically able and determined candidates will be able to take those routes. It may be that the OP is one such person, but we don't know about his or her academic profile.
By the way, Oxford and Cambridge award BAs in most subjects, including STEM subjects, for historical reasons. BA was a rank within a medieval university (this also explains why Oxford BAs become MAs a few years after the award of the BA. The MA is a step within membership of the university). Other universities call the degree in law LLB, and some people who post here mistakenly think that law firms prefer people to have LLBs, but there's no truth in that.
People seeking to work for large law firms in the US or the UK should be in no doubt about the commitment required to obtain the large salaries paid to junior lawyers in such firms. My partner's son has recently started at a big name US firm, in its London office. He works from about 9am until about midnight or 1am on the following day pretty much every weekday, and sometimes at weekends.
Young people can handle this, for a bit. When I was a young barrister I sometimes worked for eighteen hours a day and sometimes slept in my chambers with my gown or my coat as a blanket, but that was my choice and these days I choose to work a lot less. You don't always get the same choice in a law firm. This is not to say don't go for it, but be aware of what you are getting into.

I would concur this, it is definitely not a walk in the park.
Re-reading OP's initial query I acknowledge that money cannot be the sole motivator for this path (very quick way towards severe burnout). But liking the general attitudes of the US which I presume to mean wanting to work intensely hard, the fast pace and global impact are still valid reasons.
Life is hard enough as it is, so some encouragement is still highly valuable (if it may be overly optimistic).
They will likely figure out all of what you have said above on their own, which I find is the point of living life and learning.

You make it sound like someone can just waft in to Oxford or Cambridge, waft into an Ivy League Law School, and then waft into a large law firm. The routes which you mention are intensely competitive at every stage and only the most academically able and determined candidates will be able to take those routes. It may be that the OP is one such person, but we don't know about his or her academic profile.
By the way, Oxford and Cambridge award BAs in most subjects, including STEM subjects, for historical reasons. BA was a rank within a medieval university (this also explains why Oxford BAs become MAs a few years after the award of the BA. The MA is a step within membership of the university). Other universities call the degree in law LLB, and some people who post here mistakenly think that law firms prefer people to have LLBs, but there's no truth in that.
People seeking to work for large law firms in the US or the UK should be in no doubt about the commitment required to obtain the large salaries paid to junior lawyers in such firms. My partner's son has recently started at a big name US firm, in its London office. He works from about 9am until about midnight or 1am on the following day pretty much every weekday, and sometimes at weekends.
Young people can handle this, for a bit. When I was a young barrister I sometimes worked for eighteen hours a day and sometimes slept in my chambers with my gown or my coat as a blanket, but that was my choice and these days I choose to work a lot less. You don't always get the same choice in a law firm. This is not to say don't go for it, but be aware of what you are getting into.

I would highly recommend that any student who is potentially interested in a City law career reads The Times' article "I’m 26 and earn £140k. My ‘perks’? Burnout and bullying bosses" from 13th November. Yes, it's anecdotal but it adds some valuable colour to the recent Legal Cheek research on working hours.
Recognise that this career path can be brutal. The hours are ridiculous and the pressure can be overwhelming. At a junior level (i.e. trainee, junior assoc) you have no control over your hours and work/life balance, so the fantastic pay becomes an irrelevance. A big monthly paycheck is of little consolation when you're trundling home at 1am in a taxi, having cancelled (again) your hot date/drinks with friends/time at the gym/catching-up with family.
I seem to remember I shared some thoughts in a thread on the Legal Careers forum on billing targets and what that means in practice. If anyone's interested, I can dredge that up again.

All fair points but let’s keep this in context. I was a junior associate in early 2k and the hours were brutal and the money not as good as it is now. If you have the mental strength and physical endurance you will be fine.
The stress levels from a City career are not comparable to working a minimum wage job and struggling to feed your kids (I would imagine) or working in the police, hunting child abusers.