The british don't totally use the metric system... Just saying. You'll still see people weigh themselves using stones and public road speed limits are still measured in MPH. There are still "gallons" of milk in england as well.
That being said, conversion factors aren't THAT big of a deal. While it is nice to be able to instantly know that a kilo-unit is 1000 units, the average person simply does not perform that sort of conversion all that often. The only people that really care are scientists. (And yes, I do think the imperial measurement system is retarded in many ways. In fact, I really do believe we should switch).
The only think that isn't totally retarded in the metric measurement system is the temperature measurement scheme. Celsius is just as arbitrary as fahrenheit. The difference being that one picked water and the other brine.