As stated, you are on TT non-LLU, which delivers up to 8 Meg, the fastest your line will handle on that particular platform
However, if you were to migrate to an ISP providing the BT Wholesale WBC ADSL2+ platform, shown on SamKnows as:
21CN WBC status: Enabled
then might get connections speeds up to 24 Meg, depending on your current router stats which you were asked to post so we can advise you. There are many such ISPs, such as BT, Plusnet, EE and others.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC