I think Bedford has the same climate as Paris. I’ve visited DLP in all four seasons and generally never been too badly affected by the weather there. That park doesn’t have too many adjustments for poor weather. I visited Tokyo Disneyland in the winter and it snowed - it was quite magical. Key is to wear appropriate clothing I guess. I’ve lost more time to torrential downpours from the summer storms in Orlando than other parks I’ve visited.
I really hope they don’t stick an ugly roof over the top of an entire land, like Tokyo’s covered Main Street which I thought was grim. They can make sure that all queues are undercover and the majority of the rides are probably going to be indoors on any case, so that deals with most issues. They could also have some covered routes for navigating around the park, like DLP has. Beyond that make sure there are plenty of reasonably priced lockers at the park entrance where you can store some extra clothing layers if the forecast is changeable. Beyond that, I don’t think it’s a big issue.