A pay-as-you go system is probably technically possible but you've got to remember that the BBC uses the license fee to provide much more than just a couple of TV stations, Here's a list:
BBC1 costs you �3.60/month
BBC2 costs you �1.47/month
BBC3, BBC4, CBBC, CBeebies, BBC News 24,BBC Parliament & BBCi total �1.02/month
Radio 1, Radio 2, Radio 3, Radio 4 and Five Live total �1.01/month
One Extra, Five Live Sports Extra,BBC6 Music, BBC7 & BBC Asian Net total just �0.10/month
43 local radio stations (+ Radio Scotland & Radio Wales) total �0.64/month
All of the regional content on TV costs �0.89/month
The BBC web site (the largest collection of web-based information in the world) costs �0.35/month
And �1.00/month goes to bodies like the TV Licensing Agency and to provide the technical facilities to keep the transmitters on the air.
Nobody likes paying the licence fee but I think that it's worth it just for Radio 4 and BBC7 !
Chris