Travel Insurance

If you do not have a home in U.K. as you say you are living in spain then under the rules of residency you appear to be resident and also tax resident in spain.   If that is your situation then you should register on EU citizen register ASAP. 

That way your problem with  getting travel insurance should disappear

Resident (renting) not tax residents for tax purposes until January next year, hence leaving the country before 183 days are up. Anyone?

If your centre of activities is in spain. From what you say that is you as you do not have a home anywhere else. 

In those circumstances the 183 days is irrelevant.  You are resident  now.  In fact you have been from the first day you were in that situation

If you still do not believe me, and i have been  assisting people when they go to the foreigners documentation office at a national police station, for 20 years,  then take a look here

https://www.blevinsfranks.com/news/blev … ence-spain

PLC59

When you asked about residency last year I posted this

‘
-  ‘'However, if one IS MOVING TO Spain, as opposed to coming here for  a stay of up to 3 months permanently or less than 183 in total in a year,   then they must apply for EU Citizen Registration, in effect as soon as they arrive.    That cannot be started before they arrive in Spain''

According to what you have posted YOU MOVED TO SPAIN

Also.  If you did so before the end of the Spanish tax year,  ie before 31st dec 2017 you may also be liable to Spanish CGT if you sold a property in U.K. in 2017