Congratulations! If an engineering is your immediate goal (or a possibility), be careful with Williams. There are no engineering departments, and you'll have to spend an additional year or two to get an engineering credential to qualify for jobs. If your goal is a PhD, you cannot go wrong with Williams! It is highly prestigious among graduate programs, one of the top producers of physics PhDs per capita, and one of the top producers of Apker Scholars (the most prestigious undergraduate distinction in physics). The latter is the most impressive; Williams producers more of them than every LAC and most top universities, suggesting they're drawing some of the strongest physics students in the US. Research opportunities are plentiful and the liberal arts experience- outstanding teaching and a strong residential community- is one of the best in the country.

I go to Pomona as a molecular biology major and I studied abroad at UCL for molecular biology. While the experiential parts of the experience- venturing through London- were awesome, the academic experience was lackluster. There was basically no student/professor engagement, tests made up 50-100% of the grade and were more focused on regurgitation than anything else, and the teaching was poor- I had to learn everything on my own. There was a lot more independence required, which was fine as a junior, but I could have imagined it to be overwhelming as a first year. I don't know what Imperial is like, but if it is anything like how UCL was, I would be sure glad I went to my LAC (definitely appreciated it far more after returning)! You really should look at the two distinct educational models and see which is a better fit. It's very difficult to explore different fields in the UK system, whereas Williams gives you two years of exploration and then lets you specialize. Keep in mind this is how many of the top universities in the US also work- Harvard, Yale, U'Chicago, Columbia, Stanford, etc- they all have a liberal arts focus encouraging exploration of different fields in the primary years leading to the culmination of a major. I am confident that the other responses would change if it were a question of HYPS vs Imperial, but you're basically considering the LAC equivalent of the former. It is highly prestigious, and as far as I can tell, physics graduates are doing very well. Good luck!