My UK car blew its head gasket in the summer when I was based in Toulouse in the early 2000's, so on a return trip home, I took a full set of tools and spares through Heathrow to fix it.
I was travelling on my company's aircraft, in uniform, (in case I needed to use the jump-seat), and Security allowed my hand luggage with all my tools through, except my newly purchased metal scraper for clearing the areas where the gaskets seal against.
I still remember my French landlady's shock when she saw the stripped-down and disassembled engine in my car in the stable..." Oh my god ! what 'ave you done ??" Don't worry, I'll have it all back together and working again soon.
And I did
(Rover K-series engine).