During the run of the Proms there are very few other "Classical" concerts in London as compared to the rest of the year. It could be argued that during the Summer there are more people in London (UK residents on holiday, foreign visitors) and therefore the series fills a significant gap. You could also, of course, argue the reverse that more UK residents are on holiday and therefore do not require the Proms -audience sizes seem to put the lie to that one.
There are more Summer (I use the word broadly!) UK music festivals but how many in and around London major on the orchestral repertoire? Yes, there is the "official "Edinburgh Festival" but that is broadly based on most artforms and not just the orchestral repertoire and itself is heavily subsidised by the City Council and commercial sponsorship. All paid for at the bottom of the chain by you and I!
The BBC licence fee is always a hot topic and increasingly high up the Governmental agenda mainly because they have little control of how that particular "tax" is used. Advertising pays for Commercial broadcasting and who might I ask ends up paying for that - as always the general public through the the price paid for products and services. If the Public wants something they will pay for it one way or another.
One rumour currently doing the rounds is that the 2013 season will be crunch time for the Proms when you would imagine the only real savings could be made with the length of the season being severely cut. However, if the Proms are relatively cheap broadcasting there would need to be large savings elsewhere (orchestras; Radio/TV channel(s) and associated staff support etc) to make any real economic sense.
Should this line be taken the accusations of cultural vandalism on the UK's and London's music life as well as the BBC would be huge. Surely the reactions to the Radio 6 and World Service debacles show what could happen?!