That's what I thought, Joney. In particular, in earlier, more dangerous (for Catholic) times, I imagine that they showed their face at the CofE church because you wouldn't want to be spotted as a dissenter, would you?
But to maintain their Catholic faith they must also have been involved in Catholic ceremonies ----
I am not sure that Catholics would have been seen as "odd" or as having "strange religious views" (as a previous poster suggests). Some parts of England had quite high recusancy rates and the C of E is
technically a
Catholic church, though reformed.
The early threat to the state did not come from the everyday ordinary Catholics but from the landed gentry/wealthy Catholics. This was because they had the means to conspire (treason) with a Catholic enemy, like Spain for example.
I think everyday English Catholics just kept a low profile and probably saw themselves as related to the "new" C of E and understood that if they behaved as a loyal citizen, and most were, and did not make a big fuss about it, they would be left alone. Many might have seen the C of E service/rights as a good second best or at least good enough as a compromise. Adaptable, if you like.
I have three entirely separate branches of Catholics, from Yorkshire, Lancashire and Warwickshire. They all seem to intermarry on and off with C of E. They mostly seem to be buried C of E. Some have Catholic notations in the register, some don't. Some baptisms I cannot find at all. The Warwickshire bunch convert to C of E at around 1870. The children are all baptised C of E on the same day, around the time of this conversion. The others remained Catholic