If there is no "official" way possible you could consider an "unofficial" way: leave a letter with instructions to your wife on how she can access your account, close all positions and transfer the cash out. Of course these steps should be taken before informing IB that something has happened to you.
Edit: this does not only affect UK accounts. I live in Asia and had the same concerns. I asked IB for advice, but was not very impressed by the options they had to offer.