The idea of asking the INSS is a good one but, as you say, the original poster couldn’t get a clear answer, and I don’t think the average civil servant would be able to explain exactly how the two pensions are calculated. You would need someone higher up who actually deals with it. But nothing lost by trying…
I am in a similar situation and have been investigating. The “official” source you are looking for is the EU regulation that covers social security coordination: EC 883/2004. That describes the system the UK and Spain must use to calculate pensions.
I have also found people on two other forums who have actually retired and claimed their pensions after accumulating contributions in the UK (both compulsory and voluntary) and other EU countries, including Spain. And the bottom line is the system can only be to your benefit. It’s important to note there is no single European social security system or single European pension. Whatever happens you get two separate pensions, paid by two separate countries. EC 883/2004 was simply designed to coordinate and honour contributions in different countries in the case that you don’t have enough qualifying years (such as fewer than 10 in the UK or 15 in Spain) as you would otherwise lose entitlement to that pension. But the minimum you would get is what you are entitled to in each country separately.
First, each country calculates what you should get according to your contributions under its system. And then it does a theoretical calculation as if you had paid your contributions in the other country into its system but disregarding any overlapping periods. From there it calculates a pro-rata amount. If there is a difference in the two, you get the higher amount.
For example, I retire in Spain in 3 years. I will have a full UK pension (with 17 compulsory years and the rest voluntary contributions) and 24 years in Spain. So under the EU regulation, to calculate my pension, the UK would disregard my Spanish contributions as they would make no difference to my entitlement to a full British pension. Meanwhile, in its calculation Spain would disregard my voluntary UK contributions as they overlap with my Spanish compulsory contributions, and simply look at my 17 years working and contributing in the UK and my 24 years here in Spain. This could possibly, though not necessarily, result in a higher amount than my separate Spanish pension, but it would not be less. It is complicated as each country has a very different system for calculating a pension under its own rules.
That is what I have discovered from people’s practical experience and knowledge, and by reading the regulation and a summary thereof. And the DWP has also told me this is how it works. So I am proceeding on the basis of that information. And, to be honest, the idea that Spain would really deny someone a pension despite 24 years paying self-employment national insurance is surely absurd.
In any case, I have already bought my UK voluntary contributions, so it’s somewhat academic for me, what’s done is done. I’m offering no advice, but the voluntary contributions in themselves are an absolute bargain and the best investment you can make. But it’s a decision for you, one to make soon as you have until April next year and they are saturated so best not leave it until the last minute.
My only doubt now is about the different retirement ages and whether I can or should take my UK pension first (the DWP says I can)…but I have time to look at that.
By the way, if you do decide to pay voluntary contributions, call the future pension centre first to make sure which years will improve your pension (call them at 8am UK time, not a minute later), and then apply to HMRC online via CF83 (not by post) to pay the cheaper class 2. I did the whole thing in 3 weeks, which was amazing, although they take some time to actually update your record. But as long as you are in the system...
So you say you have essentially 2 options. Either get 2 options for pensions - a full UK one and 24 years of Spanish one both separate.Or, a Spanish and UK mix to give you 37 year Spanish full pension ( meaning you lose your 8 years of voluntary contributions and get 17+24=42, 5 years xtra as maximum is 37 years in Spain). So if the full Spanish pension is more than 2 separate you will have paid for 8 years of unnecessary voluntary contributions in UK plus have made 5 years of contributions in Spain that are effectively lost. Is this the case? So surely the 2 separate pensions is going to be the best as then you get pensions based on all your contributions?
And this is the worry the other poster had. That mixing pensions means that Spain discounts your voluntary contributions if the are for the same years you are working in Spain
In other words if someone has a full pension of 35 years in Spain but ONLY 13 years ( paying vast automino rates) you won't be able to get 2 separate pensions . If you worked 24 years in UK but paid 11 years voluntary contributions in UK Spain would ignore those 11 years and put your 24 years onto the 13 in Spain to give you the required 37 years for a Spanish pension. So you wasted money paying for 11 years voluntary. So surely things are not as easy as they look?