It's a bit late to change your ISP in time for Christmas, but BT has gone ahead and kicked off a last-minute Christmas sale anyway. At least this way you should be able to get a new connection pretty soon into the new year.
The sales have already started today, will be running until the 16th January, and means you can save money on your monthly internet bill. Assuming you go out and order the Superfast Fibre packages anyway.
The cheapest fibre package is the Superfast Fibre Essential, which normally costs £30 a month. If you order, you'll be able to save £5 a month on the package, which roughly equates to 17% off the monthly total.
The more expensive Superfast Fibre is normally £36 a month, but the sale means you can get it for £30 a month instead - saving you £6/20% off the total. Superfast Fibre 2 would normally cost £40 a month, but now it's just £35 - saving you £5/17% on your total once again.