as for the Custom-Fit part, perhaps it would be advisable to ask beforehand what that means and how it's achieved. For a true custom fit with trousers/jeans/pants etc, there's really only one way and that's to fit a toile, adjust the pattern and then make the real thing. It's a 2 day workshop so it's possible for example toile & fiting on day 1 and making the jeans on day 2.
The listing says that she expects you to have cut and basted the fabric before you come in, so I expect that a fair amount of time will be spent on fitting.
That said, with stretch denim at least, you're not going to know how they really fit until you wear them for a day. That was my experience anyway. I basted my jeans together, and they looked and felt fine. Once I finished sewing them up I realised that the fabric really bags out throughout the day so they're not as tight as I wanted them and there are more wrinkles than I thought. The zip also comes down whenever I sit down, and the yoke is too big, and the crotch length is too small and the rise is too short and I've got a bit of a sway back. Some of that is because of the fabric, and making a toile wouldn't have helped unless it was out of the exact same fabric. Some of that is just the learning curve of making my first pair of tightly fitted trousers. They look like jeans though, and I'm very proud of them. I enjoyed the experience so much that I know I'm definitely going to make another pair, but probably from a different pattern.
I don't expect anyone to come out of that workshop with a pair of amazingly fitted jeans. The only way that would be possible is if they were all using the exact same fabric that the teacher knew well enough to know how it behaves, but they're not. I don't personally think paying £400 plus fabric and travel costs for a pair of jeans that aren't going to fit you perfectly makes economical sense. But that's just me, and I know that I've spent money on things that don't make sense to other people, so I can't really judge I suppose.