There are a number of far east based manufacturer's of test and measurement equipment who will manufacture and brand the products for different outlets in different countries. Like some of the above examples indicate this is also sometimes the case in our country. This especially relates to lower cost products. Instruments like Voltage Detectors, Clamp Meters, Multimeters, Socket Testers are generally perceived with a market value that doesn't make it possible to manufacture within the UK or Europe for that matter.
The testers that appear identical are very likely to be made by the same manufacturer, while still being priced differently. This could be affected by the cost for the re-seller which is often controlled by the quantitiy purchased, as well as the desired profit margin which might alter from one company to another. However, it is worth mentioning that the after sales service and guarantee that is provided with this one product might also alter from one company to the other (i.e. if the company provides extended warranty or calibration services with the equipment they might not be able to match or beat the advertised price of another company purely moving the box and processing payment).
Most of these instruments are still of decent or good quality