Her husband is diplomat the same way as 80% of people working on Russian , U.S., Chinese or any superpower embassy. He is NSA/CIA personnel, probably spying on british citizens. I would bet that allowing his wife to NOT be investigated is a plain security measure more than anything else. They will let politicians to do what is necessary, but she will disappear for a very long time.
Not justifying her acting at all, but fatal errors like this will always happen until we will have two systems (RHT and LHT). This is the only reason I have never been to Scotland on my motorcycle.. One stupid error and game over.. no thanks. Not gonna risk it. And I do not consider myself stupid or not capable to handle LHT. But the brain is hard wired one way or another, for most of us.