If it's in Westminster Abbey, space will be limited.Not many celebs, I'd say. I'd expect something ultra-traditional, so no equivalent to Candle in the Wind.
Family (and there are lots of them)
Staff from the Royal Palaces
Representatives of innumerable charities and the various regiments/ships the Queen was associated with
UK politicians - all the ex-PMs and current Party Leaders, for a start
Heads of State from all over the world.