OK, looking at some Ts&Cs. Here is from the standard Ts&Cs from 12/2008.11.3. If your broadband package is not available at the new address you will be automatically moved to another available package which is provided at the same price or the next lowest price.
I have now found the legacy April 2010 Terms to which the OP is probably subject, here:
http://www.o2.co.uk/assets2/pdf/Broadband_terms_2010...
It has changed the House Move "available package" term to:
11.3. If your broadband package is not available at the new address, you will be automatically moved to another available package which is provided at the nearest available price.
However, I would still argue that if that price is an increase to your significant disadvantage, then you have a get-out free under §7.4(c) of these legacy Terms.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU BB => 2010: Orange 16 Meg LLU BB