Pensioners tax

First, you are legally obliged to pay tax in Spain on your worldwide income, and you are also legally obliged to submit a tax return for your first year of residency, whether or not any tax is due.

I'd also point out that the Spanish and UK tax authorities can look at the information held one one another's systems, so if Agencia Tributaria want to, they can easily see what your income is in the UK, then check if you've submitted a tax return here, and see if that is accurate.

The only exception is if your private pension is in respect of government employment - civil service, armed forces etc - but even then it has to be declared in Spain as that income will affect your rate of tax here.

Any tax paid in the UK is offset, in whole or in part, against your tax liability in Spain.

The sooner you take the necessary steps to get out of paying tax in the UK, the better - tax paid in the UK isn't recoverable by the Spanish tax authority, so you'd be doing your new country a dis-service if you don't get out of paying tax in the UK.

Without knowing full details of your income, I couldn't tell you in any detail, but for the average person with both a DWP pension and a pension in respect of employment, tax here is a bit higher than in the UK. But, of course, that's offset by the fact that council tax here is so much lower. And if your private pension is paid as an annuity, you only pay tax here on a percentage of that income, and at a lower tax rate than on general income.

In your situation, I'd plan on doing your first Spanish tax return next April-June, on your income in calendar year 2017. 

You need to register with Agencia Tributaria as a tax resident, and your NIE then becomes an NIF, with the F meaning Fiscal.

After submitting that return you can get out of paying tax in the UK.

A word of warning - from my experience from doing tax returns for ourselves and for friends, every tax return I've seen that has been done by a fiscal, lawyer, gestor, etc, - even people who purport to be experts - has had at least one serious error in it.

I'd be willing to post full details of the tax system here, and how to do it, but again in my experience, the majority of people don't seem to be willing to go the DIY route. Daft really; even if the person they employ to do their tax returns does them properly, you still have to pay the cost of having them done. I'm not going to spend time preparing 'how to' guides unless I think people will use them.

For your first return, you have to fill out a lot of personal information, but for subsequent years, you don't need to do that. And for the majority of people, you only have two or three boxes in the tax return in which to enter figures about income.