Eh? What's the problem? Just don't cancel your current connection! The subs for 4 months can't be much more than £49 and you won't have any delays getting a new connection. Or just migrate to O2; it should be even cheaper with an O2 mobile.
No ISP is gonna give you special rates to join but not use the connection.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU BB => 2010: Orange 19 Meg Tweaked / 16 Meg Untweaked LLU BB