There are two options when it comes to the Sky takeover. Either you wait to be migrated on a rolling contract basis, or you attempt to negotiate a better deal which will include a new minimum contract period. The trade-off for migrating to Sky on a rolling contract basis is that you will likely give up your best discount opportunity.
This reduces the choice to one based on whether you want to stay with Sky or not. If you want to stay with Sky, negotiate now - you can always reject the offer and migrate on a rolling basis. If you are not prepared to commit to Sky but have no desire to leave now, you may as well wait to be migrated to the Sky network, with your options open to bail out for another ISP.
As your current line rental is with BT, you can use the
BT Wholesale availability checker (mentioned earlier in the thread by BatBoy), which should show the cabinet number if you are connected via a cabinet, also it may show future availability information and projected speeds for FTTC. Knowing whether you're likely to be sticking with ADSL for the next 12-18 months or whether FTTC is likely to be coming soon will be useful in deciding on a strategy.
There are many good ISPs in the 'pay a bit more for a better grade of service' bracket. I have been with Zen for the past ten years - they now have unlimited fibre packages on a modest price uplift from the mainstream consumer ISPs, and I expect them to launch unlimited ADSL packages in the next few months. The only significant omission in their service is the lack of native IPv6, which has supposedly been under development for some time but there is still no sign of a launch.
You should also bear in mind that there are plenty of users satisfied with the mainstream consumer ISPs. It may well be that the likes of PlusNet offer all you need at a lower price than Zen or IDnet.