As we keep going back to this topic, it surely depends on how much is left on your mortgage and how much of a savings pot you already have/or are adding every month. E.g. if you have £100k left on your mortgage but are starting from scratch with the savings pot then it won't help half as much. 5% on a few grand ain't much!
EDIT: I thought I'd try MSE to compare the above numbers with a £100k mortgage remaining for 5 years @ 1.93%. I've probably done this wrong but here goes. Saving £500/month versus overpaying £500/month.
Saving £500/month then overpaying the £30k in a lump sum Nov 2027
Overpaying £500/month
So overpaying reduces your term and interest paid. I'm sure I've done that wrong, someone with a bit more time today can correct me.
EDIT: I didn't take into account the interest accrued when saving so I imagine you'd stick your lump sum in earlier no?
Say you took out a 200k mortgage for 25 years (assuming you had a 2% interest rate that whole time), that would give you a monthly repayment of £848 with a total interest cost of £54,312.
If you were to overpay by £500 a month from the start, this would give you a monthly payment of £1,348. After 14 years and 3 months you would have finished your mortgage and paid a total of £29,948 in interest. This is a saving of £24,364 of interest payments vs not overpaying.
If you were to save that £500 a month in a 5% gaining account for 14 years and 3 months you would have £124,328 having made £38,828 in interest. At the 14 year 3 month point of not over paying you would have a mortgage balance of £98,324 having paid £43,282 in interest. At this point you can pay off your mortgage with your savings pot and have £26,004 left over. The interest you paid minus this surplus would mean your effective payment on interest was £17,278, making you £12,670 better off vs overpaying.
There are ofc many other factors to personal finances that need to be taken into consideration as well as timing your products to avoid ERC etc, without even considering the assumptions here on constant 2% cost vs 5% gain. Purely from a maths perspective its worth doing. It just illustrates that as long as you receive a higher % return on your investments vs your mortgage you will be better off.
Inversely lets look at it from the view of a person over half way done with their mortgage from the above example. Say you have £98,324 left with a 10 year and 9 month term. That'll have you paying £848 a month with a total interest payment from then of £11,030.
With a £500 a month overpayment taking your monthly payments to £1348 you would finish after 6 years 6 months. Having paid £6,598 in interest.
If you were to save that £500 a month in a 5% gaining account for 6 years and 6 months you would have £45,972 having made £6,972 in interest. At the 6 year 6 month point of not over paying you would have a mortgage balance of £40,634 having paid £9279 in interest. At this point you can pay off your mortgage with your savings pot and have £5338 left over. The interest you paid minus this surplus would mean your effective payment on interest was £3,941, making you £2,657 better off vs overpaying.