Yes. Your Watchlist will remain saved in your BritBox account even if you cancel your subscription and resubscribe later using the same account and login details.
Managing your Watchlist is entirely up to you. Titles you've added will remain there until you manually remove them, whether you have an active subscription or take a break from BritBox.
The only exception is when a title expires, is removed, or is retired from our content catalog. In these cases, it will no longer be available to watch and may also be removed from your Watchlist.
You can pick up where you left off with your saved Watchlist whenever you return.