I believe that mains plugs in many European countries are completely symmetrical. You don't know which lead is "neutral" (essentially 0 volts) and which is "hot" (full 230V).
Here in the US we had a whole generation of consumer radio and TV receivers which were "hot chassis". One side of the (symmetrical and indeterminate) power mains cord was connected directly to the chassis/ground. They were required to be completely insulated from the user with non-conductive enclosure, plastic knobs, etc. In the case of TV receivers, the antenna connection was transformer-isolated from the rest of the internal circuit.
Even with a "hot-chassis" device, I would strongly consider connecting the chassis directly to green-wire safety ground, and working out which side of the mains cord is "neutral" vs "hot". That would provide protection to both you and your test equipment and the device itself. And using an isolation transformer would be even better.
He's in the UK which has polarised three pin plugs.
The problem with connecting a hot chassis to the protective earth when it's also connected to neutral is it will most likely result in tripping of the RCD (GFCI). There will still be a few hundred mV to 1V between the earth and neutral conductor and connecting them together would cause a fairly high current to flow between them. The RCD would see a difference in the current passing through the live and the neutral conductors and trip as a result. Another thing to bear in mind is connecting the neutral to earth conductor, anywhere else, other than the substation or where it enters the property is prohibited under the wiring regulations.
There's a lot of myths surrounding isolation transformers. They can provide both a reliable and effective method of protecting against electric shock. The have advantages over RCDs/GFCIs, as they have no moving parts and equipment will carry on running, even under fault conditions which is essential for life support equipment. On this other hand they have disadvantages: the secondary can float at high voltages with respect to earth and RCDs/GFCIs have no effect when an isolation transformer is in the circuit. A fault indicator, a device which alerts you when once side of the secondary becomes earthed, is a good idea when working with isolation transformers and in some permanent installations is indeed mandatory.
Personally, I often like to work with an earth free workbench. I run the 'scope from one isolation transformer and the device under test from a different isolation transformer. Even when the 'scope's ground is connected to the device under test, I'd have to touch both the scope's case and the device's live conductor to get a shock.