Fixing all at 1.95% definitely lowers your rate but the 10% UB overpayment is a nice option which helps with faster payment and may ultimately lowers cost
I'm not sure if Avant offer the ability to spilt mortgages across the various fixed-rate terms but you could lower your blended rate and still have an explicit prepayment option. The aim would be to fix what you can't touch but have it role onto variable about the time you can pay it off.
Such a calculation could be tailored to specific details. However for that you'd need to provide mortgage term, loan amount and how much you'd comfortably be able to overpay.
However, as a general example, why not fix 70% (20% for 3 years, 20% 5 years, 30% 7 years) and keep 30% variable.
In such an example your weighted average mortgage rate would be 2.12% (70/30 @1.95%/2.5%) in year 0. You could overpay up to 30% in the first 3 years (i.e., the variable rate element). In 3 years time the balance on the 3 year fixed (something slightly less than the original 20%) would roll onto a variable rate. You've 2 years to overpay that before in year five a further 20% rolls off fixed.
As an alternative to overpaying you could try and refix - not sure if they offer that ability and you'd also be at risk to a rate increase in the interim.
In the example above your blended rate would steadily rise (as total mortgage goes down the share that's variable goes up) given the current rates on offer you should still be below your UB rate.
If it's really any better going to all this trouble rather than just fixing the whole lot would depending on how much you could be overpaying. However, the general point is you wouldn't have to tweak much to get similar flexibility at lower rates