It depends on the employer - many companies have contracts with their clients which require a CRB check (and credit check depending on the type of data you'll have access to). I've been CRB checked by several employers due to these requirements, tends to be an anti-fraud measure. Every company I have ever worked for has asked if I have an unspent criminal record, but not all of them checked. A criminal record is not necessarily the end of your career anyway (my old manager was a former armed robber, for example) but I can't speak for every company's policy.
Whilst you haven't said, D&D and substance abuse are usually dealt with by cautions/fines, it's unlikely it was a sufficient severe penalty for it not to be considered 'spent'.
You'll usually be asked to declare unspent convictions (standard CRB check). If your convictions/cautions are spent, you don't need to tell them, and they won't show up on a standard CRB check. If they're still 'active', you do need to tell them, and the same if they (for whatever reason) require an enhanced disclosure.
Although not everywhere CRB checks on entry, some companies audit their staff's backgrounds on a regular basis. My old employer had a team dedicated to doing just that, to catch out anyone who had lied about qualifications, criminal history, etc.