I did this in 2018. I can't remember the actual number of days but I normally stayed under the 90 days and then returned to the UK and then back again. Basically I was spending just over 6 months in the UK and just under here. On this occasion it was longer here but with a trip to the UK with my wife in the middle so splitting the visit into 2 visits under 90 days. The day before we were due to leave her sister had a stroke and died shortly afterwards so the UK trip was cancelled. I never gave it a thought and it wasn't until I went to check in for my flight that the check in staff told me and it took a few seconds to work out what had happened. I had to pay a fine and got a red stamp on my passport. By the time I returned I'd had to replace my passport due to wear although I still had it with me. I had no problems but I don't know what would have happened if I still had the old passport but I assume that immigration would have a record of it.