On a fixed deal it would be difficult, you've effectively agreed to pay £xxxx per month for N years (normally 2,3,5 or 10) and are now saying you wish to pay less. I suspect they'd potentially entertain something if you could prove serious financial issues like a debt management plan. Without that you're only option would probably be to move to a different mortgage product, internally this may not be allowed or difficult to do (I guess this bit is lender dependant) and moving to a new provider would almost certainly trigger an ERC which could be thousands (or tens of thousands).
If you are reaching the end of your initial fixed period (2,3,5 or 10 years) then it would be very simple, especially when moving to another provider. You wouldn't pay an ERC and would just apply for a 25yr mortgage with someone else but this would require a new affordability check.
There is an argument for keeping the mortgage length as long as (sensibly) possible and overpaying to make up for it. Ie after your first 5 year fix you have 'done' 5 years of your 25 year agreement so would naturally have 20 years left. You move to another provider and borrow the remaining amount over 25 years again rather than 20. This means the monthly payment will be a lot less (say £300 less), you then continue to overpay this £300 each month and the 25 year mortgage would finish in 20 years as originally planned but if you had a bad month or some financial issues you could not pay the extra £300.
Some lenders (Nationwide for example) keep all your overpayments in a 'buffer' and allow you to borrow these back without any additional checks or issues. So doing the above for 2 years would give you an overpayment buffer of £7200 which you could borrow back with a simple request - making something like an extension or home improvement much easier.
