You can migrate, BT can't stop you doing that, but they will probably ask for a fee because you would have left while you were still within your minimum contract period.
Your only other choice is to get out under the "unfair terms in consumer contracts regulations 1999", the next time BT either hike up their prices or make any other changes to the contract which would in any way leave you worse off, then in those circumstances BT have to let you go free of charge should you wish to leave.
You just have to keep an eye out for any changes.