Although some Eurostar trains run from Amsterdam to London via Brussels, there is a problem regarding passport facilities. Please read this info:>https://www.seat61.com/trains-and-routes/direct-eurostar-london-to-amsterdam.htm
I would suggest that it is Seaford that you should go to to see the Seven Sisters - rather than Eastbourne. A pay on the day return from London (VICtoria) to Seaford (SEF) is £28.40 (after 9.30am on weekdays/anytime on weekends).. However, it also possible to buy a Southern Railway Daysave ticket for £19.50 (not valid before 10am on weekdays). Please read about it here:>
https://www.southernrailway.com/tickets/ticket-types-explained/daysave
For London (Kings Cross) to York & York to Edinburgh, it is cheaper to pre-book (ideally about 11 weeks ahead) specific trains. Check out the fares for your likely dates at:> www.nationalrail.co.uk
Surely, this would be cheaper than the Britrail Pass?
Now look at the map:>https://www.google.co.uk/maps/@50.7687421,0.1233618,4540m/data=!3m1!1e3!5m1!1e1
Seaford station is marked by the interlocking red arrows. You would need to walk east along the coast towards where it says ‘Cuckmere Haven’. The Seven Sisters, are the cliffs E of Cuckmere Haven - so you view them from west of Cuckmere Haven.
It is possible that you may also have time to visit Brighton on the same trip - if so inclined. If you don’t have the Daysave ticket. you would have to purchase a ticket that covers the link to Brighton.