I did the velcro trick with both my illegally spaced personal plate (but full size letters), and my wifes slighty tweaked personal plate. (Both have a legal set in boot ready to swap) But we have never needed to (Shes had hers for 8 years, Ive had mine for 6).
They only time the the velcro causes problems is when its REALLY hot and the self adhesive backing becomes tacky and can slide.
I did get caught on speed camera and to add to the pain, they also fined me for the illegal spacing (11mm between numbers and letters instead of 33mm) :O
My wife had someone crash in to her (she stopped for a police car on blues and the person behind didnt.) The police had to stop (obliged to) - and they were more concerned with her plates than the knob who'd gone in to the back of her! She played the innocent woman and they never took it further but did warn her to get new ones.
When I went to put my shorter ones on the Clio they looked silly with the recess being made for full size so I just put normal ones on.