Unfortunately, this won't be something Vodafone UK will be able to advise and this means as you are a Vodafone ES customer, you are going to need to contact Vodafone Spain Customer Services with your enquiry.
When you are roaming on Vodafone UK as you are with Vodafone ES, the only way to top up out of the country is online using a card registered in the country where you have the account.
Unfortunately even though both have the brand name 'Vodafone' in them the two are not linked.
If you run into issues and your phone is unlocked to all Networks then you could pick up a Vodafone U.K. Payg SIM card from the shop and they'll get you up and running on a suitable tariff and advise all the ways you can top up. And any relevant Calling / texting and data costs.
Or if your phones not unlocked you could buy a cheap phone that'll accept a Vodafone SIM card while you are here.