I have done this several times and never had a problem getting CP or TRI TRE signing it......as usual pilots don't really care much about what HR say, most HR people don't know the law. It is being honest because when you write it yourself you have the exact dates employed from and last day you left and your job title, that is all you need and all they are required to provide. Some companies will send a questionnaire to previous employer with the Q would you employ this person again? on it, a sneaky approach but a professional company would ignore it or write not applicable. Usually they just put the dates and job title.
For your security check and ID pass this is the information you need, no one provides or is interested in wether you did your job, went sick, or didn't show on time, these are all things the lawyers don't allow. Other items are not acceptable for security.