Your fixtures should have a maximum thermal power rating, as long as you don't exceed that with your bulbs, there should be no safety problem from the fixture.
If you were to use light bulbs with electronics such as CFL or LED, you would also need to insure you don't exceed their temperature ratings. CFLs, for example, are usually made to be used in a "base down" position so the heat will rise and dissipate. A CFL used in a recessed fixture will be in a "base up" situation where the electronics will be hotter.
Have you considered using a PAR E27? I don't know your fitting size but a PAR20, 30, or 38 will probably work out better than trying to adapt a GU10.
If the bulb fits in the base, it is the right kind. It could be a problem with the fixture, but it could be anything else along the line as well, a bad switch, bad connection somewhere, faulty insulation, chewed by rodents, etc. RCDs are very sensitive, it doesn't take much current leakage somewhere to set them off.
If a careful visual inspection doesn't reveal anything, about all you can do is take various components out of the system until you find the one that was tripping the RCD. For instance, test the fixture by removing the wiring and connecting the wires to a simple keyless base with a light bulb installed. If the RCD does not trip and the bulb illuminates, then you know the fixture is at fault.
If the RCD still trips, then the fixture is not at fault. Next try taking the switch out of the system by wiring the switch wires together so that you have an unswitched circuit. Test again. If the RCD still trips, identify intervening junctions and connect your test light at each one, starting at the switch and working back towards the distribution panel. You will eventually make a connection that does not trip the RCD. Then you know the bad section is immediately upstream from the successful connection.
I've left out many safety details here in order to outline basic trouble shooting. If you are at all uncomfortable working on building electrics, do not do any of this, find someone that knows what they are doing. Also, if you have an older UK ring system, these confuse most people, you need to find someone familiar with this rather unusual arrangement.
Best Answer
Since you have no measured voltage , I would be looking for a fuse or circuit breaker that has tripped. That bulb style in not used in my area for mains lighting. In my area I would work back to the switch then to find what feeds the switch sometimes a receptacle looking for another dead device that could have caused the open circuit or no voltage continuing to work back to the main power panel verifying that any branch fuses are good.