UK resident with matured Italian postal bonds (joint and single name)

[Medioman]

Hello all

I have been a long time lurker on this forum but this is my first post. I have been unable to find information on Google unfortunately.

I am a UK resident since 2015 but born and raised in Italy (now have double citizenship although am aware this is irrelevant).

When I was a child my dad invested in 2 savings bond at the Italian post office (Buoni Fruttiferi Postali) with a maturity of 30 yrs. Worth noting that there is no annual cash interest but rather interest is accrued every year and paid out in full either at redemption or maturity. These are paper based and one of the bonds had both my name and my dad's name on it while the other only had my name. (literally a piece of paper with our names written on it, date of issuance and interest). These both matured last month. I had no idea these things existed until then...

We went to cash them out at the post office in Italy over Christmas (the joint one needed either myself or my dad to be present, not both, while obv for the single name one I needed to be there) and we got back principal + all the accrued interest minus tax (Italy taxes interest on these things at 12.5% as they are aking to a government bond). I understand this is is correct as I was an Italian tax resident when the bond was issued (have already read the Italian tax authority guidance on it).

Now the question(s): when I do my SA next year what do I need to tell HMRC? Is this "foreign income"?
What should I expect in terms of tax charges in the UK? Does the fact that I already paid taxes on it matters?
Also should I declare the joint one (maybe for half of the amount?) or just the single name one?

thanks in advance