The Eurail Global Pass will be very good value as train tickets in the UK are very expensive (more than Switzerland !). All trains in the UK are covered, sometimes the Eurail tool is unreliable...

A 35€ seat reservation is required for the Eurostar between Amsterdam and London but afterwards you have complete flexibility and can travel anywhere ! Free optional reservations within the UK if you'd like, a good idea on busy mainlines such as London - Edinburgh.

You need to book Eurostar well in advance (aka as soon as your dates are set) as there is a limited number of seats available to passholders on that route. What is your travel date ? Alternative if the direct trains are full : hourly reservation-free IC to Brussels + Eurostar to London (30€) -> always more seats. Check availability more easily here : https://www.b-europe.com/EN/Booking/Pass#TravelWish

The Eurail Global Pass will be enough, no need to bother with the almost as expensive BritRail Pass. Only count how many pass days you might need.

Do not hesitate for questions.

Btw a 1st class pass does not cost much more and in the UK you get meals included on long-distance journeys (unusual nowadays).

The UK shares less of its timetables with the outside world in comparison to a couple of years ago. Almost all trains are included, which makes it a very pleasant Interrail country (with free seat reservations) and immense flexibility.

Tickets bought the day itself are very expensive in the UK! Often more than double the price in comparison to an advance ticket. So a pass will almost always be good value for a spontaneous day trip. 

If a train doesn't show up in the rail planner app, you can always add a train manually. 

For UK timetables check this website: https://www.nationalrail.co.uk/. It's the best option to plan your trip, much better than the rail planner app.