I mix it up all the time.
I cook in metric and buying food makes sense to me now in metric, measure knitting in metric and weigh my dog in metric (that's probably because that's how it's done at the vet).
I weigh myself in imperial, always think of my height in imperial and when my DD talks about what her children weight, or come to that her own weight she'll tell me in imperial.
If I go on a journey, I do it in miles but, as pp said, I buy my petrol in litres.
If I try to deviate in any way, the measurements or weights don't really mean anything, except perhaps with food.
When money changed to decimal a lot of people went on for some time still converting new pence into pounds, shillings and pence until we got completely used to the new system and we had to get used to it because it was the system we were using.
The problems with weights and measures is that as a country we haven't entirely embraced the metric system: we still buy eggs by the dozen, beer is sold by the pint, babies weights are given in pounds and so on.
Also, America uses imperial of course, so we hear it all the time in American TV programmes and films, so it continues to be something that makes sense to us.