Even if you are tax resident in Spain ( that is have an EU citizens registration Cert. sometimes still referred to as a Residencia, or if you spend 183 or more in Spain, or your main residence is in Spain) income from rented property in the UK is only taxable in the UK.
Where one is tax resident is governed by rules, one cannot just choose where they pay tax. If one pays tax in UK, say on their OAP, when they are tax resident in Spain, they still have the obligation to pay tax on that pension in Spain.
Only people who are resident in the UK can claim the heating allowance. If they subsequently move to Spain, they can usually keep getting the allowance.
But a person who, is resident in Spain(as above) who falsely claims they are resident in UK would be committing a criminal offence.
As you say Smythie, ‘the law is the law.’