Doesn't the 5-day, 5-country pass make more sense and should be close to what separate tickets cost?
In your case money you'll pay for the pass won't be final sum, since in case of travels in Spain, France and Paris-London connection there are in most cases pretty essential extra expenses associated with special pass supplements and seat reservations (like in case of Paris-London with
EuroStar pass holders fare might at the end be higher than buying from scratch one-way discounted ticket). So don't be tricked by 'raw' pass price. Although at the same time pass can be a solution, since it will allow you not to bother with in advance ticket purchase and will give you a good deal of flexibility. While with discounted tickets (which are usually non-exchangeable and non-refundable) you'll end up tied to a specific train and date. As result you just need to make decision which criteria (mainly it deducted to flexibility (with pass) vs. price (with point-to-point tickets) you prefer) are more important for you.
Just to give you the idea, here we list app. fare for discounted point-to-point tickets purchase in advance (basically to compare total with the pass price):
Barcelona-Marseilles: here you can easily count on 60-70 euro or alternatively (in case you aren't afraid to make an extra train change) you can also travel by regional trains for roughly 65 euro (and that is full fare, so you won't need to buy anything in advance, but just before departure)
Marseilles-Lyon: with in advance purchase you can get a ticket for only 20 euro
Lyon-Cologne: roughly saying from 60 euro for discounted ticket
Cologne-Brugge: full fare here from 43.4 to 54.2 EUR (depending on train), while discounted tickets start from 29 euro
Brugge-Paris: full fare here is 47.2 euro, while discounted tickets start also from 29 euro
Paris-London: discounted tickets roughly start from 50 euro, but sometimes (during certain promo campaigns might be even a bit cheaper)
Also to note, April isn't a peak season month, so you probably won't have problems buying discounted tickets even like 2 month in advance, while if you do so earlier, then you like 99% will get the best point-to-point ticket price possible