The best advice I can offer is to close-up your home as appropriate, and simply go walkabout throughout northern Spain for several months as a pilgrim. Do not do anything in the UK that is irreversible just now.
For example, avoid selling your home until and unless you KNOW that you will not need to return. The same advice goes for a car, etc. Avoid the emotional temptation to make a clean break and just go. Even so, do not shed yourself of these hard to recover assets until you have made sound investment or banking arrangements. You may think you are going to live on permanent Camino, but one still needs money.
Immerse yourself in the culture you seek to adopt as your own. Observe, take notes, ask questions. Meet people, make new friends, build a social network, rely on the kindness of others to help steer you in correct directions. This will avoid false starts and save time. Yes, you will make mistakes and have some false starts. But, doing it this way will result in less effort, expenditure, frustration and disappointment.
At Santiago, and elsewhere across Camino routes in Spain, there is an expanding network of English-speaking expats all devoted to supporting the entirety of the Camino community. Not financially mind you. You need to have your own resources.
There are a lot of folks, many of whom are available through the forum, who would likely be pleased to offer advice and recommendations. Simply ASK. They will make themselves known to you, typically via private conversations...unseen by the rest of us.
These folks, many known to me as friends, are fonts of local knowledge. They are also, to a person, very kind and helpful. They can help you navigate the maze of bureaucracy and different cultural norms that may be paradoxical to northern Europeans, or especially to folks seeking to put down roots in Spain, but coming from outside Europe. However, I think it best to leave it to you to ASK, and for the others to reply or offer their assistance, independent of me. Nuff said on this point...
While I spend up to two months in Spain and at Santiago annually, in connection with doing a Camino and serving as a volunteer at the Pilgrim Office, I do not live in Spain. As a US national, I do not enjoy the same sort of visa-free right of residence as you do (at least pre-Brexit). But, please believe me when I say that I am usually one 'bad day' away from pulling the plug, obtaining a long-stay visa and relocating to Santiago to join the Camino support network. As a UK national, you have it relatively easier. But it can be done.
Over the past six years of my association with the Camino and with being a volunteer at Santiago, I have constantly maintained a watch over living conditions, flat costs, the cost of living, bureaucracy, laws, visas etc. I try to keep my knowledge current, just in case I may someday have to 'pull-the-trigger' and head to the Spanish Consulate at Miami, or to their embassy at Washington, DC.
The key thing IMHO is to DO YOUR RESEARCH! Look before you leap. Do not let your emotions rule your brain. Seek out advice from a variety of sources. Compare notes. Visit and see for yourself. Spend time in each place you THINK you are drawn to.
Oh, and I think it goes without saying, do learn Spanish. The uptake rate of English as a second language is lower in Spain than in Portugal or most northern European countries. I suspect this is for two reasons:
(1) Prime time TV programs in Portugal are in OV with Portuguese subtitles. ALL international programming TV in Spain is dubbed. The same holds true in France and Italy. This usually accounts for the low English as a second language rate among these folks. Once you live in Spain, hearing English on the TV is a rare thing unless you cable provider offers BBC World, or perhaps the occasional football match from the UK.
(2) I was told recently that most Portuguese schools mandate English instruction at some point in elementary grades. This may explain why most Portuguese have at least some English.
Hope this all makes some sense, and is somewhat helpful..