Audio/Video: Would they actually listen to audio and video content as well as all the faxes and emails? (maybe that is their problem not yours?)

They are there to deal with their constituents so allowing mass mailing of more then one MP (ie encourage people to mail those that are not own constituency MP) may be counter productive as it will be viewed as spam and clog up actual constituent messages getting through.

Visualization on a map would lend some weight and give a nice view of the support/feeling.

Should definitely make use of some of the existing services such as writetothem.com as they are already great services and it would be wasteful to have to rebuild that functionality or regather that postcode data.