Eddie - no problem & happy to help.
Many city firms will be a bit sink or swim to some degree, but the US firms have more of a reputation for it. If you're keen on secondments it's the large London city practices that provide the best opportunities.
Getting the training contract is the hardest step, once qualified it is *much* easier to move around, particularly at the 1-4 year PQE ("post qualification experience") mark. So if you trained at lets say CMS and qualified into corporate, it would be relatively straightforward to move into a corporate team at other city firms (including US/MC etc...), and many city lawyers do just that.
Sublime's post above is very accurate on what that US law firm culture can (and usually is) like, city law firm life has demanding hours anywhere and it's always difficult to say what an average day is like, but the really high paying US firms pay you that money for a reason - and that reason is the expectation that you'll work very very hard, bill lots of hours and that work always comes first. This typically means finishing up somewhere between 9pm and 11pm on a typical day, later when its busy (which will be often), and plenty of weekend working, cancelling holidays and plans as needed to get the work done.
To put that into context what can "US hours" look like?
You're in your financing seat at a Kirkland or a Lathams, It's been a busy week at work, getting home after 11pm each night and then up and back into the office for 9am. It's Friday and you're looking forward to an overdue catch up with friends later that evening before going to see your parents for the weekend for a family birthday.
At 3pm in the afternoon an associate comes over to your desk, a new transaction has come in - you need to join a call at 5pm with the client and deal team. You join the call, it all sounds very hectic and you're not quite sure whats going on. Afterwards the associate tells you the bad news - "it's urgent and the client needs first draft documents prepared for 9am Monday morning, cancel any plans you had - we're going to be working the whole weekend."
You're set your tasks, there's a lot to be done and you get started (dropping your friends and family apologetic texts) you leave the office around 2am pretty tired.
Up early Saturday morning and online for 8am, you've been set an internal deadline by the associate to have everything ready for them to review on Sunday afternoon, you work the whole day, emails flying around until around midnight when you're able to log off. Sunday starts earlier - up and online at 7am to finish off the document drafting and you get them ready for the associate to review that afternoon. Whilst they're reviewing you catch up on some email filing and some other work you're being chased on for another transaction.
The associate drops the documents back to you at 8pm and sounds a bit annoyed with you "I would have expected these to be better..." they say. There are plenty of changes and some additional documents to prepare arising out of their review, you work through Sunday evening and into the early hours of Monday morning, dropping the updated work back to the associate at 4am. 4 hours sleep and then back into the office on Monday at 9.30am for another busy week of late nights
Now that would be a week on the busier side (although not as busy as it could be - you didn't have to pull any all-nighters!), a "quiet week" would be something along the lines of 9.30am-7/8pm and a couple of hours on Sunday.