What travel date do you have in mind?

Can be done in 4 changes, no less : London - Paris - Vienna - Ljubljana - Rakitovec - Pula

Sort of early August.

I thought I would have to go London - Paris- Munich-Ljubljana-Rakitovec- Pula

That's correct if you'd like to go via Lake Bled. 

It's also possible with 3 changes. If time doesn't matter...

How ? You cannot reach Ljubljana from London in 1 change

I've just seen that your "4" doesn't show your itinerary that has 5 changes mentioned; I thought you did count correctly. But I didn’t either.  It's one change less (that's what I did mean) so it's indeed 4 but one less than shown in the itinerary. But what would be your itinerary with four changes as this one shown has 5?

But that doesn’t matter anyway if the OP wants to go via Lake Bled.

At the moment, I can only find a journey to Munich changing at Stuttgart as well. Then if I search Munich to Bled I would have to change twice - Salzburg and Villach.

I’m not planning sleeper trains as I don’t think they’re covered by Interail Global Pass, and I kind wanted the least clunky, changey journey as taking a ten and thirteen year old!

In general night trains are covered but you need to pay a supplement. There's a direct night train from Stuttgart to Lesce-Bled. (But the station nearest to the lake is Bled Jezero; depends on where you'd like to go concretely).

Paris to Salzburg can also be done by night train. 

London directly to Bled is a too long journey. A night train may be a good idea. Or a break in between.