Some of the most popular axles fitted on trailers in the UK are made by Al-ko, Knott, Avonride, BPW, Peak and Indespension. It is possible that at some point a replacement may be required, identifying the existing make is most important to start with. Usually, a plate is riveted to the centre of the axle with the manufacturer's name.
It's worth looking at the pictures and diagrams of the Alko standard Hotline axles on this page, paying special attention to the brackets. If your axle looks like any of these then it can be reproduced much quicker and cheaper than others. You will just need to take a few measurements from your existing axle.
Should your existing axle not look this any of the Alko axles listed then all the manufactures can match an axle to the serial number of an existing axle. Ideally, we need a picture of the axle plate so this can be forwarded to the manufacturer.