If you are a UK resident you only have 2 travel days in the UK, including the Eurostar, but I hope you know that already. 

I don't think that it is very common that trains sell out in Sweden. It might happen around Easter, midsummer and Christmas. There is mandatory reservation on the high speed trains, the IC trains and on Snälltåget. On the line Stockholm-Gothenburg Interrail is only valid on the SJ trains?, not on MTR and Flix.

Seems you have prepared yourself and read-good! Then you must have noted that we daily declare that ES is the very worst #1 in helping pass-holders-all trains have to be REServed and on the populair lines-somehow any newbee seems to want to use just these- sold out days before in hi-season=now.

FR is #2-but thats for its TGV and awkward way to REServe-other as in ES its most often possible in case of really ´all full´ to go via slower local=TER trains. Last yr I had a pass and was in FR last week of aug-when all go back to home-but I still could find RES-seats on every trip I wanted-maybe at other time, by doing it at the local machines=these give good overveiws and sometimes even alternatives not thought of.

NL has no RES-except INtern, BE ditto, DE-local trains are overflown this summer due to extreme cheap ticket-ICE CAN be REServed-no need, and have a dreadful ontime record-falling by the week till around 50%-with also many cancellations. 

IN general-when school hols are over-this varies from mid-aug till late sept-mostly the further south, the later-it calms down a lot. Except that then there is fairly often the business-peak-on fast trains-but noone knows how this will be post-covid compared to past.

Thanks, yes aware of the 2 day restriction. I’d travel out to Europe and then come back at the end of the trip.

So Spain is the worst, but it sounds like only then it would normally be a few days in advance that reservations would sell out? Not weeks? Is it still possible to take local slower lines as an alternative? Is there ever a time when the interrail pass would just be completely useless?

So a month later I’ve been checking again and reservations for London - Paris, Saturday night in 2 days time are available. So that’s good - one doesn’t have to wait weeks to get a eurostar, appears to have been a height of summer thing/abberation due to flight prices.

It's never been so bad as this summer. Usually you will find some availability a few days in advance. Also note that the Eurostar from/to Amsterdam seems to sell out rather quickly but London-Brussels much less so, even on the same train. So when London-Amsterdam is sold out, you can often book London-Brussels and then use the hourly IC service (reservation-free) to Amsterdam. When there's no availability on London-Paris, you should also look for seats on London-Lille, since it's the London-Brussels service that stops there and availability can be different.