I think it's going to be a mixed bag. As it will help BT with Home Broadband (Home Broadband in UK is not up to the par of Mobile networks). Yes, it might have negative effects on Mobile consumers.
There are still 4 operators in the UK not 3. Instead of EE/Three/O2/Vodafone it'll be BT/Three/O2/Vodafone.
No big deal really, if anything should make things more beneficial to those who want quad play services, as instead of just Virgin offering this it'll be BT too, so multi-service discounts are likely available, or better retention deals.
Business knows that the more service a customer has with them the less likely they are to leave.. it'll just be interesting to see how things change within BT/EE - ie > Will they offer a proper 'capped' service so customers can't exceed their allowances, and will they allow tethering as EE do at the moment..