Any upgrade will be to 21CN, but there is no schedule for that on your exchange. When & if they do Orange should follow suit.
Have you considered TT? They are LLU'ed there and should supply "Up to 24 Meg" ADSL2+. I personally don't recommend them as it will be hard to get your phone line back to BT when you leave, but they might give you faster speeds
providing your line could support them.
Do the current occupants have BB? If so, their router stats would show if the line would benefit from ADSL2+.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 15.6 Meg LLU => 2011: Orange 16.8 Meg Untweaked WBC