I do not know anything about the Trust Fund voucher so do not know if you can get it out before a certain time, but if you have to leave it until he is 18 things could change drastically in the 12 years and may be worth a lot more.
It sounds like the Halifax are offering a bonus for 12 months, read the terms and conditions as you may have to give notice before removing the money. You should then be able to look for the best deal at the time and put it with the new Bank.