Theres plenty of options to forward UK numbers to a Spanish phone. Have a Google search. Your customers dial the UK number and pay at local rates. The call is forwarded to a Spanish number and you pay the redirect charges. (Claim as a business expense)
The business part is reasonably straight forward. Consult an accountant/Gestoria for the best option. Ask the Gestoria if they are a member of the Collegio de Gestorias. They are qualified for EU tax advice.
You can retain the UK ltd and become a Spanish subsidiary/office with you as an employee. IE: you pay employee social security in Spain, draw a wage and pay in to the Spanish tax system. The UK company pays the office expenses, issues invoices etc.
Alternatively become Autonomo. Self employed in Spain and invoice the UK company as a contractor.
Both options require a Spanish EU VAT/IVA number. Theres no collection/transfer of VAT for business to business trading across the EU. Even after Brexit there’s no VAT collection/transfer trading with a none EU country.