Possible to extend the fixed rate term for a mortgage

[Manlove is my forte]

A lot of mortgages don't allow overpayments or set it at a low limit like £500 per year.

Well mine does and allows me to pay as much as I want as long as I do not pay the whole mortgage off within the first 3 years (like thats ever going to happen)

Either way, once the 3 years are up he can swap lenders. If he saves the money (instead of paying directly into the mortgage) he can invest this into his house at that point. £400 a year for 30 months is £12,000, not a small sum by any means.