It's not 'no taxes' flights vs 'full fare' flights.

Every flight has a mixture of numerous individual ticket prices, and I'm pretty sure that any 'no taxes' offer is limited to a certain number of qualifying passengers, beyond which the only direction is to more expensive fares.

So the answer to your question would be no.

Having said that, all Ryanair flights to/from STN are pulled today, in addition to all other carriers to/from LON from 11:00, because of the volcanic cloud heading this way.