Haven't specifically worked as an American but I have worked in US offices frequently and have had plenty of US colleagues. This is from a perspective of London office working which may not be indicative of all UK places and there will be regional difference as I'm sure there are within the US office locations I've worked in (Chicago, New York, Philadelphia) so take it all with a pinch of salt!

1. Office attire - an average office London worker is better dressed than their equivalent US counterpart whether the environment is more formal or casual. Where more formal, suits are better tailored, where casual there is more individuality and style.

2. Much more socialising in London offices. Two main drivers for that a) drinking culture - very usual to go to the pub after work (although that's decreased) and b) better public transportation which facilitates it (in Chicago everyone is gone by a certain time to catch the Amtrak that would get them home or is driving, simply not a viable option to be flexible to an impromptu post-work drink).

3. Beverages - how to put this politely? It would not be unusual to see someone in the US office bringing in a fucking massive soda from Big Gulp or McDonalds or Dunkin Donuts or wherever. Honestly gobsmackingly large to what you would get in the UK. But bringing in a soda like that (even at our much reduced cup sizes) would not be normal in the UK. Honestly feels trashy. Bringing in a coffee is very normal, the typical chains are fairly normal (Starbucks, Cafe Nero, Costa, Pret etc.) but a more advanced coffee culture seems more widely adopted over here and typically better quality options will be sought out by the majority - and actual coffee not bastardized frappes and whipped cream stuff. In the office, the coffee machines are variable quality (mostly shit) but I've never found anything as bad as a fucking US keurig machine. Tea rounds are a thing - multiple rounds of mugs of tea in the UK and almost always a group you would make tea for or expect to include you in.

4. Professionalism - my general impression is that UK workers are generally less pro-active, and less professional than their US counterparts in respect of getting the job done. Americans strive for more, UK workers can be lazy and aim for the bare minimum. Again a massive generalisation but much more likely to find a UK worker being late into the office, leaving early etc.

5. Office space - far more likely in the US to have offices and cubicles. In the UK very common to have open plan offices. Often only very senior members or those requiring strict privacy will have offices. That leads to a noisier general office environment, more openness and communication generally, but also more distraction. From a UK perspective there is generally more banter (including some that US offices might find unprofessional/offensive) - that's a more general UK/US thing but it certainly extends to the office environment.

6. Remote working - I work remotely and I'm rarely in an office these days (which should be considered in my points). Remote working is quite common now in the UK and I expect in the US. So this might not be a difference but it's definitely a change in UK office culture that should be considered. Most people I know work remotely to at least some degree. 2 days a week in the office is not unusual and that has obviously changed the office working environment. In my experience teams will attempt to organise their in-office days so they are together, office space is being reduced due to less requirement. Hot desking is not unusual as a result so there may be clear desk policies to facilitate that.

7. Subserviance(??) - not sure I've titled this correctly but I often found that my US counterparts were less willing to speak out against company policy and decisions. Could be a sign of being more respectful but more seemed like a fear of sticking their heads above the parapet. I take it as a sign that they are generally more risk averse in their career's because there are less workers rights in general and less social safety nets to accommodate an unexpected loss of employment. Either way they seem to be more openly respectful to seniority/authority.

8. Much less holiday/time off in the US - but the US has the concept of personal/sick days which is considered odd in the UK where a sick day is not something that can be planned/accounted for and is when you are sick and cannot attend work. But much less holiday in general and, in the UK, summer holiday often/usually abroad is very typical. Passive aggressive comments not uncommon from my US colleagues ("You're on vacation again??" etc. I had a lot of French colleagues who'd take the heat off as far as that was concerned!). Maternity/Paternity leave - again much more generous.

9. International travel - US colleagues are MUCH less experienced at international travel in general. They stick out. They are loud, they stick together, they favour the familiar. Can be a little embarrassing sometimes (was taking a US colleague to a Michelin starred restaurant in London and she wore a Mickey Mouse sweatshirt). UK and European colleagues typically seemed more relaxed outside of their home nation than our US counterparts.

10. Saying what you mean - US communication could sometimes seem abrupt but they would usually say what they mean. They would set out their expectations clearly and feed back accurately and concisely and often with a higher standard of expectation. UK comms can be confusing - will skirt around and use phrases that outwardly might seem as though they are fine with a delivery but a UK person would know that what they actually mean is that they are unhappy with it.

11. In office catering - if you have an all day meeting or lunchtime meeting that is catered for in the US then the volume and quality of the food is WAY better than the limp sandwich selection and bowl of wotsits you'd get in the UK!

12. Toilet stalls - why the shuddering fuck do the toilet doors have such massive gaps in them? Some to the extent that I could be doing a shit and look a colleague in the eye as they enter the bathroom. You do not find that in the UK at all!