Hi Suzy,
Yes you will have to make a change in Paris, so it makes a great place to stop off. The most efficient route is London - Paris - Barcelona - Madrid, and each of those cities is overflowing with things to see and do, so you could break it up and stay in one or all of those places that you need to change.
For 5 days of train travel within 10 days rail passes start at about £234 for an adult pass, and that would make sense if you wanted to stop off on the way. It will most likely be cheaper than buying point-to-point tickets as well, but for some trains you will have to pay an added seat reservation fee.
Hope that helps!