You are not automatically considered non resident by the UK Inland Revenue, you need to contact your last Tax office and ask their advice. The Tax office in Nottingham generally deals with overseas residents and claim for tax exemptions, but there are exceptions. When you have a certificate of exemption you can send this to your bank who should then stop deducting tax. However, you DO have an obligation to declare Tax, RENTA, in Spain, EVEN IF IT IS A NULL DECLARATION or you are intitled to a refund. If you do not speak or read Spanish you should seek professional advice, otherwise simply go to your nearest Agencia Tributaria after May 1 and pick up form D 101 ( Simplificado) and fill it in. You will need the end of year Fiscal Certificate that your bank will give you and all the details of your property etc, including catastral number and value. When you come to a total that your either owe or are entitled to a refund, you take the form to your bank and pay.
Many banks will help with filling in the forms. Don’t forget there is a double taxation agreement with the UK and any pensions etc taxed in the UK are exempt from tax in Spain. As a Spanish Resident you have to declare income on WORLDWIDE assets. As a Resident your minimum level for paying Patrimonia ( Wealth Tax paid by residents and non residents) is hugely increased, this may take you out of the bracket for Patrimonia.
Don’t listen to people in the bar who are always experts! If you are in the tax computer as having purchased a property, if you have a bank account and a fiscal (NIE) number, it will catch up with you eventually so do make sure you tax affairs are in order.