You’ve chosen the two most challenging interrail countries in Europe though, France and Spain. TGV (High speed, long distance trains between the big cities) can sell out some days, on popular dates even weeks in advance. A bit of planning is necesarry, when wanting to take TGVs. Count an extra seat reservation cost too. Without seat reservation, you won’t get on any TGV.
But challenging, does not mean impossible. TGV is always a good option, on longer distance. But a bit pricey and unflexible.
The only reservation you really need far in advance is that one of the Eurostar (London-Paris or London-Lille, and back). Get it as soon as possible.
In France, you can travel reservation free, but you’ll be solely using TER (Train Express Regional). Not requiering any reservations (or a very small fee, TER NOMAD - to and from Normandy).
Use www.bahn.com to plan. Go to further options in the planner, click Type of Transport and unselect “High Speed Services” (and maybe Intercity trains).
Barcelona by regional train is also possible, passing by Perpignan, Cerbère (Fr) Portbou(Es) with TER and Rodalies (avoiding a very expensive seat reservation on the infrequent TGV to Barcelona).
Note that TER usually do not have 1st class, except in the Grand Est Region (around Strasbourg), nor any catering options. Although I find the TER seats rather comfortable, especially compared to other regional trains in Europe.