So I'd probably go with one of the cheaper deals (it looks like they don't charge an arrangement fee, which is nice), stash the extra cash somewhere that earns more interest than the mortgage is costing and then decide later on whether you want to dump it into the mortgage as an overpayment or not. You can easily get 5% interest on £300 a month using one of the regular savings products offered by a variety of banks: http://www.moneysavingexpert.com/savings/best-regular-savings-accounts
The Halifax site is a bit quiet on what conditions are attached to overpayments, just some mentions of early repayment charges so check that you can overpay without penalty if that's something you might want to do, although at under 2% interest you're probably able to do better by using savings to hoard cash until the time comes to switch deal - you can probably pay off a chunk of the mortgage then if you wish (probably only a good idea if interest rates have skyrocketed or it lets you get into a different LTV band).
Looks like from the Halifax website that if you overpay, you can claw the overpayments back by underpaying at a later date:
https://www.halifax.co.uk/mortgages/mortgage-information/overpayments-underpayments/
So I'd probably go with one of the cheaper deals (it looks like they don't charge an arrangement fee, which is nice), stash the extra cash somewhere that earns more interest than the mortgage is costing and then decide later on whether you want to dump it into the mortgage as an overpayment or not. You can easily get 5% interest on £300 a month using one of the regular savings products offered by a variety of banks: http://www.moneysavingexpert.com/savings/best-regular-savings-accounts
The Halifax site is a bit quiet on what conditions are attached to overpayments, just some mentions of early repayment charges so check that you can overpay without penalty if that's something you might want to do, although at under 2% interest you're probably able to do better by using savings to hoard cash until the time comes to switch deal - you can probably pay off a chunk of the mortgage then if you wish (probably only a good idea if interest rates have skyrocketed or it lets you get into a different LTV band).