Off there myself! There are some good tips on the Royal www or there is stuff on YouTube which gives a pretty good run down of what happens:
YouTube - Queen Elizabeth investiture ceremony
I'm told that many good hotels in London do "special" packages for people attending investitures - and details may be provided by Lord Chamberlain's Office. As for after BH celebrations, it's up to you - good slap up lunch (Ritz, Savoy, OxO Tower or somewhere ilke that gets my vote although RAF Club is very traditional) and then change into planters for a complete pi$$ up!!! As for best up to date gen, I'm planning to look up someone who I know who's been recently and get the low down.
Have a great day!