I think Mark and Martin cover the key areas here but I’ll mention a few things anyway.
Flights with Icelandic air from Gatwick to Cleveland via Reykjavik is a good way of going. Not bad at all and think they are pretty reasonable as costs go.
May or August are the best times to go, but I would check calendars in advance for both parks as the May still be closed days.
2020 for a new coaster is likely, given the park’s anniversary.
You will probably need two visas, one for the Ststes and another for Canada.