They are probably going to have to do so to keep up with the other LLU ISPs and now that BT are doing so with 21CN they are going to appear to be lagging behind.
In actual fact they should have no problem switching over to adsl 2+ because the technology works.. and Tiscali have been installing adsl2+ MSANs in "their" exchanges since 2005.
AFAIK they use the Huawei MA5300 in all of the exchanges where they have a presence so its actually pure and simple a case of actually giving the higher speeds to their customer, and no "trialling" as such needs to be done.
ATM the restriction they place is purely at the ISP end. Take for example how they limit the tiscali consumer upstream to 448kbps thats at the ISP level.. whilst other ISP's who use tiscali wholesale have been able to give their customers the higher upstream since the very early days. In fact I'm pretty sure that Tiscali wholesale customers via other ISPs were getting the 8Mbps speeds long before Tiscali ISP customers were.