Hi, the good news for you is that UB allow overpayments on fixed rate mortgages, before checking if a break fee is applicable.
From memory they allow 10% of the Mortgage amount to be repaid each year. You should avail of this rather than earning 0% on your money.
I missed the date in your first post. All of the banks only extend their cash back offers on a rolling 6 month basis or thereabouts, so nobody will know what cash back offers, if any, will be available then.
You should keep an eye on rates / offers available, and see if it's worth breaking your fixed rate early if rates go lower.