Omnibuzz has been trading for over 10 years and we are always hearing of some rogue 'operators' in London area. There are few things that give these people away:-
1) The web site does not have a postal address
2) The contact number is only a mobile or 0207 or 0208 then the next digit is 1 or 0. Physical land lines are 0207 or 0208 then 2-9 as first part of end number.
3) Price quoted is under £550 for double deck unless an explanation given of it being 2nd job of the day so bus is already in town.
Routemaster buses are not cheap to run, tax and insure plus wages for two crew for a day, even if you only have it for short time. Ask the company to provide you with copy of their bus insurance and public liability cover.