My father went to the UK a long time ago on a 6 month visit visa. But, he ended up staying and working there for about 10 years. The Home Office had taken his biometrics, but in 2008, he left the UK and took a taxi to Spain. Since 2012, he has been a resident of Spain. He would now like to visit the UK again and plans to apply for a 6 month visit visa. Although it has been more than 10 years since he left the UK, should he reveal it in the application where they ask if you have ever stayed or worked in the UK illegally?
Yes, your father should tell that he has been illegally in the UK if asked about it on the forms.
Never lie to an immigration officer, not in person and not in forms, as the results of being caught in a lie are very strong.
Usually, when a lie or deception is uncovered, the resulting action is a ban, which may be temporary or life long, depending on what is lied about, and can influence the option for getting entry to this and other countries for a long time.
Visa forms often have the question if you have ever been banned and if so, you have to answer yes for the rest of your life, even when a ban has been long in the past and is no longer active.
Whether an illegal stay in the UK will make it impossible for him to enter the UK legally now will depend on several things, among which how he left and how strong his ties to his current life are.
If he is settled in Spain, with strong links to his life there, his chances are much better than if he has no links and no reason to return to Spain. So read up about 'ties to your country' and such on this site. And prepare as strong a visa application as you can. Or if he wants to stay in the UK and work there, apply for a visa allowing for that, with all supporting paperwork he can find.
Never assume that because something is long in the past it has been forgotten. And immigration officers are very good in finding out lies in applications.
Better be honest and hope for the best.