As everyone has said it should be fine, anyone can own pc's etc.
BUT
I'm gonna say something contentious that not everyone may like, but it has to be considered.
You haven't said where your coming from, home country or ethnic origin, & Unfortunately some government agencies have been known to use racial profiling as a tool to essentially narrow down their search process in the "fight against terror".
In that respect, should you tick any of the stereotypical boxes they look at, as someone here on a student visa, it would crap if some over eager civil servant spots the "military vest", & misconstrued it's purpose.
If you felt this was feasible, might be better to mail the vest to your UK address & avoid any unpleasantness.
On the flip side, probably like many here I like to use certain brands in daily life, whether it's clothing or luggage etc, when travelling to certain countries I've been inclined to tone down my attire, turn up looking like a poster model for 5.11 etc & you may get mistaken for something your not.
It's a worldwide problem, but all we wanna do is pew pew lol.