The UK has only allowed the US to use their air bases to strike Iranian offensive capability and intercept missiles launched towards middle eastern cities. Iran bombed an airport in Kuwait yesterday, for example.
It's pretty obvious how the the UK's actions vs. Iran's, or even the US's, are different.
Yeah, "striking offensive capability" of a country is aggression and the country that strikes, and it's helpers are all aggressors and in the wrong as far as intl. law goes. You need to work on understanding how causality works. What happened yesterday has no bearing on what happened 2 months ago.
If UK/US wanted to be in the in the clear they could have asked UNSC to authorize use of force against Iran.