Do you know how much fat was in your meal, sushi and crisps combined. Fat can delay the BG rise/spike post-meal. So if there was a high fat content in your meal, this could explain why you were okay after one your and then 18.1mmol/L two hours after.
The other reason is that your hands had not been washed prior to the test which gave you the high reading? If you have traces of foodstuffs on your finger tips then you're readings can be severely exaggerated, this is why washing of hands is recommended. It's partly due to hygiene as well of course. Did you re-test after your 18.1mmol/L reading to establish whether it was a wild card?
It is possible your BG got that high, and if it did - then it could be time to start monitoring your carbohydrate intake closer and look for lower GI alternatives to what's causing these huge BG spikes.
Good luck!
Grant