Healthcare

If you have residencia then you are tax resident.

If you stay in Spain for 183 days or more in any year, you are tax resident in Spain. You don't have a choice. 

Did you take out private healthcare to gain Residencia, or did you rely on reciprocal state healthcare ? 

If you relied on reciprocal state healthcare, a no-deal Brexit looks likely to blow it away at the end of 2020.

Not according to my Solicitor. We are Residents because we spend more than 90days here but not Tax Residents as we don't spend 183+ days here

We took out Private Healthcare but obviously this is expensive and I wanted to explore what we would be entitled to in Spain

Your case is interesting.

It seems that, by gaining Residency in Spain, you can circumvent the post-Brexit "90 days in 180" constraint, but that you have no plans to exceed 182 days in any year (thereby not becoming liable for income tax in Spain).

On that basis, your effective Residency stays in the UK, as does your state healthcare. 

You purchased private healthcare in Spain in order to qualify for Residency in Spain. I am aware that some people gain Residency using private healthcare, and then let the private healthcare lapse without making alternative arrangements. I don't know the legalities of this (perhaps others can advise) but it is short-sighted.

It seems likely that EHIC cards will become invalid under any no-deal Brexit. In any case, EHIC cards provide for emergency treatment only, not for repatriation, or coverage for chronic conditions. 

At a practical level you could (a) continue with your Spanish private healthcare or (b) let it lapse and rely on extended holiday insurance. Depending on your ages and any pre-existing conditions, (b) will probably cost less than (a).

Under option (a) you won't get repatriation to the UK in the event of serious illness. You might get better/more prompt access to treatment than you would under the UK NHS (my wife can attest to this). If you enter a treatment process in Spain, you might not be able to return to the UK as planned, so you would then become a Spanish tax resident by default. Any unplanned protracted stay in Spain could have other consequences (e.g. for your car).  But if you do develop a "condition", and keep your private healthcare current, the "condition" continues to be covered.

Under option (b) you might need to seek repatriation in order to avoid becoming a liable for Spanish income tax. Additionally, if you develop a "condition", that "condition" becomes a "pre-existing condition" when your insurance needs renewing. 

If nothing untoward happens, you retain UK NHS coverage under both options. 

The path that my wife and I took was a bit like yours. When we bought our home in Spain, we were uncertain of how much time we would spend here. We purchased private healthcare and gained Residency because we didn't know what Brexit would look like. Theresa May's deal had not been published at the time, but we knew that we needed to do something if we wanted to stay in Spain for more than 90 days at a stretch. As it happens, we like living in Spain; we were coming to the realisation that Spain is our home, and that we should sort out our tax situation, when my wife became seriously ill (something untoward did happen). She entered a protracted treatment process in Spain in February 2019, and will not be able to travel very far until 2020. So whether we like it or not, we became liable for income tax in Spain. 

FYI, if you have healthy private pensions (in addition to your UK state pensions), your income tax liabilities will be higher in Spain than in the UK.