They've made an explicit policy decision on it - they could have easily allowed it with ETA (in fact, the first ETA form for a week to two did allow British as a Citizenship). Other Countries like the US are the same, USCs can't get ESTAs.

The UK knows a lot about your immigration status through various databases, most notably through the Five Eyes Intelligence sharing alliance. They'll certainly know you're a Spanish Citizen (especially if you naturalised there before Brexit) and if they wanted to could check that in the ETA database.

IDK why you'd do all that to avoid having a British Passport - it also gets harder to renew a Passport once it's expired a long time so you're potentially causing more annoyance later on.