So if I'm reading all that correctly, the line in my area is capable of 512kbps, and I'm getting about half of that down and a bit more than half up, which I suppose is due to my distance from the exchange (I'm about 5 miles from Stevenage) or Sky intentionally slowing me down.
Those figures are the reverse of what would normally be expected. The "A" part of ADSL (Asymmetric Digital Subscriber Line) refers to the fact that the design provides for different transfer speeds in each direction, but it is the downlink speed which is the higher (on the assumption that most domestic users will have far greater quantities of downlink data than uplink).
The fact that your downlink speed is reading so low, especially compared to your uplink, does suggest that Sky might well be "throttling" your speed. Those online speed checkers will just calculate the average speed based upon the rate at which you receive packets of data. You want to access your router's configuration pages to find out the
actual data rate at which you are connected to your local exchange. That will give you an indication of what your line is capable of, free of any other restrictions.
The thing that's making me wonder is the sentence about unbundled broadband up to 24Mb being available in my area. Is this just marketing BS that doesn't actually apply to me because I'm limited to 512kbps, or is there some better way of getting broadband I'm missing out on?
If you're 5 miles from your serving central office, then there is no way you will ever get 24Mbps -- or even anything remotely close to it -- over DSL. I'm in a similar position here, about 5 miles from the C.O. as the wire runs. Quotes will generally be for the minimum 0.5Mbps service, although we can sometimes stretch it to 1Mbps or so, depending upon the particular line.
Here's some current information about Stevenage exchange, which includes the LLU operators currently present there:
http://www.samknows.com/broadband/exchange/SMSXIt also notes that Virgin Media provides cable service, but as your results indicate it is not an option I assume that the boundaries do not extend much beyond Stevenage itself and the immediately surrounding neighborhoods.
So there's no way you'd achieve those multi-megabit speeds over DSL, and if you can't get cable, the only other way to get higher speeds would be via one of the satellite services.