Hi, I understand the desire to be housebound rather than bedbound, and I don't want to be a wet blanket, but can I ask why the plan is to move to Lanzarote? You might say for the warmth, but being indoors this winter has been quite chilly if you are sitting still - I am sitting in a heated room typing this. Most people would go and sit outside during the main part of the day in winter to warm up, but not from 6 pm to about 8 am.
As far as you person position goes, I obviously know nothing of your condition, but, I would have thought that getting social and medical support from the state would be a lot more difficult here than the UK. If you leave the UK to live here, legally, you lose your entitlement to health care, and must sign off from the UK NHS and from your GP. To become resident here, you must demonstrate you have health care provision, either by being employed, or a dependant, or by showing adequate private health care cover. https://www.gov.uk
is a good place to read up.
Your parents should know that unemployment is much, much worse here than in UK. They would probably only get the fairly physical jobs you mentioned, not managerial ones, and after the first few years would move into their mid 60s, where I am now, having to work hard without building up any more pension than they already have. https://www.gov.uk/government/world/spain
is a good source of official guidance.
On the subject of tax, I am on firmer ground to advise, because our income is based on various types of pension, and UK rental income. UK government pensions (armed forces, police, fire, teaching, local government etc) can be taxed in Spain instead of UK, as can private pensions, from a company scheme, and UK rental income. It's your choice, but takes a little organising either way. The UK State Pension (Old Age Pension) MUST be taxed in Spain, NOT in the UK. The good news is, you can get personal allowances in boh countries (if you pay tax in both) The bad news is, if you want to be strictly legal, Spanish tax affairs are vastly more complicated that UK ones, and you really need a qualified Spanish accountant. Of course, many people just chance it, and pretend they live here less than 183 days in a calendar year. www.hmrc.gov.uk
is a starting point.