Evening Ann and welcome to the forum, by the way!
So far as I'm aware, and as you have kind of hinted at, the E121 is a transit form from the UK NHS to the Spanish Seguridad Social (Health Service). I'm not sure that your logic regarding cover from the NHS whilst you don't submit the E121 is sound.
If you are resident in Spain (have you done your empadronamiento yet with your ayuntamiento?), got your residency certificate, etc - then having the E121 is good because it is one document you will require to register and receive free cover in Spain. Unfortunately, if the above applies, whether you had the E121 or not, you are not covered by the NHS back in the UK. The only people who are covered by the NHS are those who are here but officially are not - if you catch my drift - and therefore so far as the NHS in the UK are concerned, they're still UK residents.
So, the important point to your question is, where are you officially resident?? If in Spain, then get yourself registered with the Seguridad Social. All treatment covered by them for any Spaniard is free to you as a retiree in Spain. I am of course assuming here that you are of retirement age (being 65 in Spain), with exceptions that we won't go into to keep things simple.
You will get free prescriptions, full medical/hospital/specialists etc cover and the only thing you will have to pay for are, as the Spanish have to, certain specialist things like certain creams/ointments and non-essential in the eyes of the SS, but generally these end up costing less than your usual prescription in the UK anyway. My Mother-in-law these days of the new digital prescription service and who unfortunately suffers from many and complicated ailments, usually comes out of the pharmacy with the equivalent of a supermarket shopping bag full of medications and sundries. It does not cost her a penny. Neither do the heart specialist, the traumatologo, the eye specialists, the lung specialist, etc etc and many more etc's. In fact, we run a little taxi service for her it seems and are kept very busy and we know just about every specialist and hospital unit in our area.
You will read in several posts in this forum that the general consensus is that the Spanish SS is an excellent service. However, if you can fund it and don't want to wait around for appointment waiting lists, then of course private medical is also available to you - should you wish to take the additional cover. It's rare that pensioners do that in Spain - as with the above mentioned plus emergency services and doctors on the phone etc, it's rare, if not unheard of, that anything urgent gets left for any length of time. If and when the time comes, you even get home helps to clean your bathrooms and get you ready in the mornings, all on the SS via the ayuntamiento. This is certainly the case where we are in Galicia and I'm sure must be similar with other medical services throughout the country.
One thing's for sure - if you are resident here and please bear in mind that I mean that in the full official sense - not just living here and having not bothered to say anything to the UK, I'm guessing you will have done as you have the E121 - then registering with your local GP and getting their admin to sort out an SS card for you is a win/win situation. Let's face it...if something urgent did crop up, you wouldn't want to rush back to the UK, just to find that it was indigestion, would you?!
Tallulah.x
