The UK does not share criminal record info with any other countries or the EU (which isn't a country, obviously). There is no link between your UK passport and your criminal record, and any information put on the Schengen Information System by the UK police was removed as a part of Brexit (not relevant to you but maybe relevant to others). So, if you choose not to disclose, they won't know, but if you travel without disclosing under ETIAS, if you ever need a Schengen visa, you will need a police certificate, which will show your record, and they would then see that you had travelled without disclosing at a time when you should have disclosed, which would give them a decision to make. For now, though, until ETIAS starts, you do not need to disclose.

I know I'm replying to a super old post but I've only just seen this. My wife has had to get a Schengen visa a few times (she is not an EU or UK citizen) and she's never had to get any police details on herself. Yes she ticks a box to say she doesn't have any convictions but the visa is more concerned about financial status and if you have a valid reason to return to your port or origin.

There are no criminal record questions on the harmonised application form, probably because criminal record questions are asked if someone applies for residence in a particular Schengen country. It does seem strange, though, that we will be asked the question for ETIAS applications but not visa applications, unless individual countries ask anyway.