Just to make things more interesting, most fixed body trailers (boxes, curtain sides etc) have the height marked on them in meters. However, bridge heights in the UK are by law marked in imperial units. As councils replace old signs they have both units on them. But there are many old ones out there. So you have the added complication of converting from one set of units to another. As an antidote to this, I carry a small credit card sized reference card with conversions in 3 inch increments. If you can’t get one of them (can’t remember where mine came from) then there are conversion charts you can download and print out. I strongly recommend doing this, and carrying it with you.
Additionally, these heights on trailers are measured with a standard 5th wheel height of 1250mm. But some units - MAN’s spring to mind - that run at a higher setting. 1350mm in this case. That’s a whole 4 inch higher than a trailers stated height! But units with non standard heights usually have stickers advising you. Take a good look at your truck for them, including on the rear of the cab.
But the golden rule is, if you don’t know the height of your vehicle, don’t take it out on the road. And be extra careful if the height of your vehicle changes throughout the day.