Ive just downloaded Flash at around 9 Mbps.
One other possibility could be a problem with one of the transit peer points from your ISP
You could try doing a tracert to Adobe.com and see if theres any problems with any of the hops, but I have a feeling that Adobe may use akami to deliver content.
Im not saying this is going to be your problem, but years and years ago when Plusnet very first started doing traffic shaping, and before they sussed out the Elacoyas, 2 of the very places that I reported pathetic download speeds were Flash and Opera downloads (I used Opera back then). I honestly dont know for sure, but I think Opera may also use Akami... which is why Im wondering about a transit link problem.