I use Telewest's 512k service so do not strictly conform to your requirements for recommendations.
However, I would say that you have nothing to lose and all to gain by switching to broadband for at least three reasons:
1. As you say a normal dial-up line will never perform at 57.6k, unless you are in a lab. Even if you are fortunate enough to get an initial rate of 50k it tends to fluctuate for the duration of the call. This does not happen with cable - if they say 128k, you tend to get 128k throughout. This is not to say that congestion does not happen - it does with Telewest but fortunately it is still a rare event.
2. The price NTL quote on their site is good, considering it is 1 Pound cheaper than AOL charges for their dial-up package, and only 1 Pound more than Freeserve does for theirs.
3. Once the kit is installed you switch to the higher throughput package with no further hardware change (at least that's what happens with TW, so I presume the same applies to NTL)
And if it doesn't work out you can always cancel and get your money back - those are your rights as a consumer.