Seljuk22 - EZY are in the process of withdrawing from Warsaw due to increased charges there. The Luton - Warsaw flights cease in early July ... so no chance of Gatwick - Warsaw.
I can't comment on Vilnius or Bucharest but as far as Brussels is concerned most people in the London area use the Eurostar to get there. You need to remember that the London - Brussels market is much smaller than London - Paris. The former is predominately business travellers whereas the latter has a lot of leisure travellers too as Brussels isn't perceived to be a deseirable tourist destination.