it seems we're getting the lower end of the scale - my line estimate is 23meg.
Does this mean the cabinet is quite far away from my home?
Yes - This page
has a table that guesstimates you as being around 1km from the cabinet.
(How can I find out where my cabinet is and the line length - I do know from using the BT Wholesale checker that I'm connected to cabinet 46).
Use BT Wholesale's address checker to check the speed estimates for other addresses, and other streets (but checking they are on cab 46 each time). As you keep trying, you'll probably see the speeds increase in one direction, which will head back to the cabinet - which will probably be on some arterial road route back towards the exchange. When you get to estimates above 65Mbps, you're really close.
Using the address checker, you should just enter the postcode value alone, which will then give you a set of addresses to pick from - so try a sampling.
To get an idea of postcodes to try, use the Royal Mail's postcode finder, specifying just the street name and town. The street names can come from any local map, or Google Maps.
does this line speed estimate mean it's a waste of time ordering Infinity 2 as I won't be able to take advantage of the potential higher speed ceiling?
As an early adopter of your cabinet, you're likely to get higher speeds initially (with no interference from other subscribers at first). However, it still isn't likely to be beyond 40Mbps by much - in which case Infinity 2 isn't much benefit, unless you want the other extras that come with such a package (such as the unlimited allowance on Plusnet or BT)