We've had lovely holidays in the Netherlands, up around Medemblik is lovely.
Eurostar to Paris and trains anywhere else in EU continent. (Eurostar definitely goes from Brussels but I don't know if it goes to London from there).
Ireland - some gorgeous places all over and driving very easy, same language, similar food etc. And great afventures to be had - surfing, cycling, tree trekking (zip wires), hiking and climbing, sailing, swimming, go karting, all sorts. Same language cinema and theatre for rainy days.
France - Atlantic coast is really lovely with both great beaches and lots of history and French food....nom nom.
Spain - Santander region by ferry, Bilbao an San Sebastián are gorgeous.
Within UK itself - lots of interest all over and huge variety of weather, food, activities etc.