If the Straits of Hormuz are closed to all traffic then a coalition of the willing is formed, and takes part in any agreed military action against Iran to reverse the situation, on the basis that pretty much the whole world is effected. The UK contributes what it can/will to such a coalition.... (maybe we could do the catering?)
If the Straits of Hormuz are to be selectively closed - the question is how exactly? Stop and board all shipping (that won't be done using Kilos and mini subs). Turn back or impound all shipping going to destinations that Iran doesn't like? Where does the fuel used in the UK come from - does it for example include crude oil that is shipped to and refined in the Netherlands..?
Look beyond the glossy Janes and Intel briefings on sexy kit, and try to work out how/if it can be used practically in anything other than openly declared all-out warfare...
If Iran wants to take on the whole world, then using Kilos and mini subs to sink everything in the Straits might be a viable plan. If Iran doesn't want to take on the rest of the world.....
Using the Iranian scenario as justification for the MRA4 is grabbing at straws to put it mildly, which is not to say that there aren't more valid scenarios that could be used.