The Eurostar leaves from London (St Pancras station). From there, there are different ways to get to Spain depending on where you want to go. For Madrid or Barcelona, the simplest way is to take the Eurostar to Paris, which takes only 2 hours and 2 minutes, and then book an overnight train or 'train hotel' to Madrid or Barcelona. From there you would get a local connection to other destinations.

You can also book a ticket for the whole journey in a lot of cases, including any changes. For example, by putting 'train london seville' I found this offer, and if you try the same thing for whatever Spanish destination you want, you may find you can arrange the whole thing online. But otherwise, if you go to a travel agent and ask them to fix you a trip from London to any Spanish destination, they can do it for you.