Further to Dzug's post:
The 757 Green Line service operates from Victoria Green Line Coach station which (somewhat confusingly) is not the same as either Victoria Bus Station or Victoria Coach Station. There's an explanation, together with a map, here:
http://www.ukstudentlife.com/Travel/Transport/ Coach.htm
Most services terminate at Luton Airport. Only a very few go through to Luton town centre. (However, there's a fairly frequent local service connecting the airport to the town centre). The 757 timetable is here:
www.tinyurl.com/greenline757
(Use the 'radio buttons' to select your required day and direction of travel and take care to note the column headings, as some services only run on certain dates).
A coach driver on the Ipswich to Stansted airport route told me that the definition of a 'local' journey (where senior citizen bus passes are valid at the appropriate times) is where there is no more than 15 miles between bus stops. If the same rule is applied to the 757 route, it would seem likely that your pass will be valid. For definitive information, phone the bus operator (Arriva Shires & Essex) on 0844 800 44 11.
To get from Romford to London, entirely by bus, you should take service 86 to Ilford and service 25 from there. It might be best to join the 757 service at Marble Arch/Park Lane (Stop 14A) or Dorset Square/Baker Street (Stop S), rather than at Victoria. To plan the whole of your journey, use this site:
http://www.travelinesoutheast.org.uk
Chris