Is thermite illegal to own in the UK?

it is illegal to make or possess more than 1 gram of any pyrotechnic composition.

Thermite is a pyrotechnic composition, so unless you get permission to use a highly combustible substance at a rock concert, I'm afraid that you can only make it in 1 gram quantities, IF you want to pay attention to the law... if you don't however... just don't go blowing up government buildings!

The previous answerer was incorrect about the amount that is legal to own. I owned over five pounds of thermite and some other pyrotechnic compositions, and as long as I used it for non-malicious purposes and keep everything safe, no one had a problem with it.

I'm pretty sure it is legal to own in the UK, as most other places. Just make sure everyone and everything is safe when you do light thermite or any other pyrotechnic composition.

However, flash powder is somewhat of an expection, since it is considered a type of explosive in some places.