That's part of the difference. My understanding is the UK approval took the company's analysis and reviewed and signed off based on that analysis.
The US review starts at the raw data and does it's own analysis.
Close, but not quite. The external (not FDA employees, and people who were not involved in the trials) advisory board doesn't have the data yet. There's a step in between where the FDA will review the raw data and create an analysis. This is sent to the external advisory board about a week before the meeting to give them a week to review the analysis before the meeting. I think we're still in that middle part where the FDA is processing all the raw data to create that analysis. The external reviewers will depend on the FDA analysis while the FDA depends on the raw data. I'm sure those people doing the analysis worked over Thanksgiving and the weekend, and probably long days too.
In the old days, they would have sent a semi (or several) full of paper to the FDA with all the raw data on it. Now they do it electronically, but it's not any less information, probably more. For every trial participant, every manufacturing step, all the way from where the raw chemicals come from through every step in production, distribution, and trial patient. It's way more than just "did it work?". But, is it repeatable and consistent, and does it work, plus all the impacts. It doesn't help anyone if production batch B is different than A, and A worked but B doesn't. Or, if they both work but B had more side effects. Things that would mean batches A and B were not exactly the same.
And, this is just for an EUA with a limited scope of the most at risk people likely to be approved to get it. Full approval for general availability to any random person is going to dig even deeper.
Lots of the analysis of drugs in general is weighing the risks vs the thing they're solving. The risk should be lower, much lower, from the drug than the alternative. Healthcare works and seniors in care facilities, the first to likely get approval, are at much higher risk from COVID which means there's more leeway. A perfectly healthy 15 year old with no other risk conditions needs a much higher bar that there's no risk. It's part of what makes vaccine approval so much stronger with less tolerance for impacts. We're giving it to someone who is currently healthy and do not want to change that. In comparison, a cancer drug could have lots of issues but the alternative is cancer kills you, there's lots of room for complications that are less bad than death.