In theory you need a RCD tester as it should trip within 40 ms and clearly that is a tad too fast to do with a stop watch. The test button on the RCD is for mechanical testing to ensure it has not stuck, and there is no requirement for the test resistor to be set to 30 mA, so although it trips with the test button you don't know if it trips either in the required time or current.
Proper RCD testers start at around £120 mark, however if you are testing yourself you also need to test the loop impedance, the Martindale EZ150 will test loop impedance however it starts at 1.7Ω and that is over the fail point so looking at around £170 for the loop impedance tester, before you use these however you should test before even turning on these will cost around £100 I have looked for cheapest I could find, not trying to scare you. It is debatable if with an insulation tester that you need the loop impedance meter as you should be able to calculate the results. But to inspect and test you are looking at £220 to get the testers.
I am sure many do work without testing, and many use the cheap plug in tester like the EZ150 at around £50. And also many only test the RCD using the test button, however this does not make it right, with a large firm with many testers you can compare the results by testing a socket in the office with all the meters, it is very unlikely all meters will malfunction at the same time, so if the readings are about the same the meters are OK. But for one person on his own he needs to send the meter for calibration, or own two so he can compare. This means for DIY really doing your own inspection and testing is not economical viable. And inspection and testing will likely cost as much as getting some one to do the whole job for you.
We have talked about this many times on this forum. And the question is should we tell people it's impossible to DIY and do a proper job or not? I think we will be lucky to get most DIY people to buy the EZ150 you can get cheap socket tester for under £5 but they do very little. So you have to do a risk assessment, is it worth the risk to DIY? I can't answer that, only you can answer that.
I looked at rewiring mothers house, £2500 to get some one in, and that includes all the materials, for me to do it need to replace my meters £500 and pay the council £100 + Vat last time I looked could be more, so that means in real terms £1500 for them to wire house and they will take one week I will likely not be finished in a month, so I got a firm in to do it, and I am an electrician. It's a lot of money I know, but wanted to be able to rent out the house if mother did not come home.
So what do you think?