http://smile.amazon.com/Amprobe-INSP-3-Wiring-Inspection-Tes...
This tester can simulate a 10A or 15A load and measure voltage drop, which should remain within 5% of its unloaded value (this is the recommendation in U.S. code). This can identify situations where connections are weak, or wires are too long or too thin.
It has also helped me to improve my wiring practices. It turns out that keeping voltage within 5%, under a 15A load on a 20A circuit, is pretty demanding, and a series of (say) 8 or 10 twisted connections may not meet it, if you are not careful with your technique.
The device can also test GFCI outlets by allowing some current to leak to ground. This provides an end-to-end test in situations where the GFCI is not present at the outlet.
http://smile.amazon.com/GE-3-Wire-Receptacle-Tester-50542/dp...
(that's US wiring but surely there's a UK equivalent)