Roller coasters can be built indoors. Universal is renowned for a lot of their indoor attractions some of which are coasters.
I don’t think our climate will hinder this, Comcast will have done enough research on this project to ensure that it’s a worthwhile investment for them, they’re not doing it as a favour to the UK. Whilst not every attraction in the park will be undercover, there will be enough there to not put people off when it’s raining, and outdoor rides don’t usually stop running in the rain only if there’s a thunderstorm nearby or it’s excessively windy.
I’m excited about this and think it will be a great income generator for the country, as well as creating thousands of jobs. Businesses in the surrounding area and even those further afield will benefit from the extra tourism too.