You don't know what your government get up to, the same as we don't know what out government get up to.
I agree that negotiation is the best recourse, but that should have attempted before you took a ship in retaliation. Retaliation leads to more retaliation, and can lead to war. IF (and it's a big if) the UK took the Iranian ship illegally, then why didn't Iran use diplomatic means, or even mediation before taking a UK ship? C'mon .... tell me why!
I find it very difficult to believe that the UK ship 'was on the wrong side' or that it rammed an Iranian fishing vessel.