Hi
Firstly, if you want to move to live in Spain from the UK, you need a visa. If you ate retired you need a Non Lucrative Visa (NLV). I successfully completed the NLV process in May 2021.
You need to apply for your visa at the appropriate embassy / consulate in the UK. Broadly speaking, London serves the south of England; Manchester from the Midlands to the North West and Edinburgh for the very north of England and all Scotland. The consulate websites, visas sections, provide the list of requirements, as per below - with my added comments in bold.
LIST OF REQUIRED DOCUMENTS FOR A NON LUCRATIVE RESIDENCY VISA (NLV)
1 Passport or travel document valid for at least 6 months beyond the intended arrival to Spain with at least two blank pages to affix the visa.
2 One completely filled and signed National Visa Application Form.
3 Ex-01 form and 079-052 fee form to be paid at the Consulate on the day of your appointment. The visa and appointment fee totals approx. £600 per person.
4 One recent passport colour photograph. (No more than six months old)
5 For non-British citizens: A UK residence permit valid for at least 3 months. It can be either in form of Visa stamped on the applicant’s passport or a Residency Card. If you are a 'normal UK subject / citizen, this doesn't apply.
6 Medical (Health ) Certificate signed and stamped by a registered doctor. This Certificate must literally state that the Applicant does not suffer of any illness which represents a risk or a danger for the public according to the International Health Regulations of 2005. The document must be translated by a Sworn Translator only, and with the Hague Apostille if the country is a member. Otherwise the document must be verified by the Spanish authorities in the country of issuance. Our doctor charged £25 per certificate, using a template provided by our Spanish solicitor.
7 Certificate of Good Conduct issued by the country or countries where the applicant has resided in the past five years. The document must be translated by a Sworn Translator only, and with the Hague Apostille if the country is a member. Otherwise the document must be verified by the Spanish authorities in the country of issuance. This is a Police Certificate which is obtained from the ACRO website, at a cost of, I think, £55 per person - turnaround time was about 10 days.
Note - docs 6 & 7 had to be notarised (our UK solicitor charged £25 to do all our documents). The Hague Apostille is an additional government service, which costs £30 per document and has a turnaround time of about 10 days.
8 Full health cover for applicant and, if applicable, family member. There are specific private health insurance policies, which must not include 'co-payments', which are acceptable for the NLV. Many companies offer these. We used Sanitas (which is part of BUPA), as our bank had a linked offer. I have seen DKV recommended by other people on this forum. The policy needs to be paid up front for a year and be 'live' at the date of the NLV application. We were 50 & 48, with no pre-existing conditions and our policy cost around £1,200 for the two of us. I have seen people with pre-existing conditions getting quotes as high as £6,000 pa each. It's important to shop around as prices vary significantly.
9 Means of maintenance in Spain of €2400 € per month for the main applicant. Each additional family member will need €600 per month. In general, €37,000 pa for a couple.
10 All documents must be translated into Spanish (and where it specifies a 'Sworn Translator', these are specific translation companies that charge a lot of money (however, our Spanish solicitor included this in her price)).
Although I would say that I am quite confident and competent in completing forms, gathering evidence, etc.; as many forms needed to be completed in Spanish (and mine is very limited) we used a Spanish solicitor (Raquel at BeGlobal Attorneys) to assist in preparing all the paperwork, making the appointments, etc. but there is still quite a bit of work you need to do yourselves. I would certainly recommend Raquel.
For the 'means of maintenance' we provided evidence of cash in bank, shares, premium bonds, rental income from a property we own and rent out in the UK, and my pension pot.
In total, the process took us about 12 weeks and cost around £2,300 for the two of us (excluding the cost of the private health insurance) - of which the solicitor's fees were approx. £1,000. When you get the NLV, you then have to enter Spain within 3 months and then start the rest of the process (below) within 1 month of entering.
When we got to Spain, we then had to attend the Foreigners' Office at a specific Police Station to have fingerprints taken; and be added to the padron at the town hall. Approx. 5 weeks after this, we collected our TIE from the Police Station.
This NLV lasts for a year, then you have to renew for 2 years, then a further two years, then you can get permanent residency. I have already had my first renewal and the process was similar ('means of maintenance' & private healthcare, but I didn't need the Police check or the medical certificate).
Regarding your pension, the following are my views, but you should always check with a professional pension / tax advisor. Your state pension is unaffected by moving to Spain. However, as in the UK, pensions are taxable and, whilst the actual tax rates are similar to the UK, the personal allowance is lower (approx. €6,500 per person - as opposed to £12,500 per person in the UK). This means that you would pay a little more tax on your pension. Private and company pensions are the same, but if you have a government pension (Police, army, nurses, civil servants, etc.), these are only taxed in the UK (but they will still count against your personal allowance for any other tax you pay in Spain).
Hope this helps - and happy to answer any questions you may have.