A pothole in the pissing rain is all it takes to do the inside wall. Hasn't happened to me (other than a bubble that you wouldn't notice 'till you or NCTS look for it) in over 30 years though... but, like yourself, I wouldn't be the hardest worker in the world by any means. I actually had a bit of luck with the NCTS this year. The inspector passed the motor and told me to have a look at the front left tyre which he suggested was wearing on the inside... when I looked (albeit a few weeks later) I discovered a bubble.
If I had a car without accommodation for a spare I'd definitely keep one at home though, as Ken suggests. In fact I have a spare spare in my shed that came with the MKII Octy... it's actually the wrong rim for both cars (the rim hole is too large) which is why it was replaced when I bought it, the dealer just didn't want it back... but it would do in an emergency as all the bolt holes line up!.