I've worked on a few big EPC projects in a specialised industrial sector, including some in the UK. Three things drive the first digit of cost, assuming you've hired relatively competent engineers and constructors:
1. (Design) Building floorplate and architectural complexity (i.e., divergence from 'big box')
2. (Construction) Schedule adherence. Almost any one-off expense to stick to the schedule is worth it, but to your point, these are often challenged or delayed. Building and testing equipment on skids off-site is almost always worth it.
3. (Design/Commissioning) Schedule adherence. For commissioning this is typically driven by design choices (did you pick a high-TRL process, or if not, do all the work required to mature it in parallel to construction?) and by building the right commissioning team (knows their job, knows the plant).
If more expensive plant & equipment gets you ahead on any of these 3, 99% of the time that is an overall optimisation.
I was once working on a one off build and also on a smaller build at the same time. The smaller build was built start to finish as per the plans, the larger one was choices all the way.
The net result, we topped out and roofed the big house, then built the small house in its entirety while the owners of the big house chose their windows. Add that in favour of waterfall design...
Not permitting and regulation? We're told that is the biggest impediment to development.
Permitting and regulation is generally a sub-component to each of the three things mentioned, and can be a major factor in schedule adherence for large scale construction projects. Whether that is the primary impediment or not is often a factor of the specifics of the project (and its design) and the overall environment. During major events (e.g. COVID), materials access and availability was a far more challenging aspect of schedule adherence for most large projects than any other factor.
I've never worked in this field as the GP, but I have family that do and I've heard plenty of stories and made my own observations, but definitely take my two cents with plenty of salt. Maybe GP will reply also.