Mortgage term ending - pay off lump sum or save it?

DDangerous

Hello. I have a 2 part mortgage. One for £143,000 recently fixed for 5 years at 3.94% (16 year term). The other part is £135,000 - fixed term and is currently at 1.94% and that runs out in February 2023 (14 year term) . I am in the fortunate position to have £20,000 I can use as a lump sum payment to reduce my mortgage (by paying off less than 10% this year, then another payment in the new year). I was planning on doing this on the part of the mortgage that expires in 2023 to reduce it as much as possible before I have to renew at what will most likely be a higher rate (our plan is to eventually sync up both parts of the mortgage so the fixed terms end at the same time and we can then combine them into one mortgage and have the ability to shop around lenders in the future).
Is this the right approach if I want to safe guard against higher monthly mortgage payments once my fixed term expires? To me it seems to make sense that reducing the loan by £20k will help offset any interest rate rise when we come to get a new product, but am I better off investing the £20k for a year in a savings account and paying off a lump sum then or is there a better approach given what is happening with mortgages and savings? I'm finding working out the savings on mortgage vs interest rates over a short term period a bit baffling:) - thank you in advance for any guidance. 

DDangerous

sorry - mortgage expires in Feb 2024 (not 2023)