Although before answering you should check if the Rehabilitation of Offenders Act applies.
Indeed; it is - in most cases - not legal for employers to ask about spent convictions, and if one is asked, one may lawfully conceal it, to the point of refusing to answer or telling a lie. An employer may fall foul of employment law if they ask, or, as a result of asking, sanction or prejudge an employee or applicant. It's in some ways even more restrictive than the Equality Act - an employer can ask and collect data on protected characteristics like sex/religion/disability but in most cases cannot even ask about employees' spent convictions even for monitoring purposes.
We wouldn't uniformly tell people that "we always advise you tell your employer if you're gay/a Catholic/trans/a gypsy" on the off chance you get a bad employer who flouts the law anyway, so the important takeaway is - read the law, understand your rights, and make your own decision. Unlock is a charity which will give you impartial and free advice.