Capitalising mortgage overpayment

LittleBrownHouse

Hi there.. 
I am overpaying my mortgage by £200 each month and at the end of the fixed term there is some overpayment amount saved.
What have I done on previous product transfers is I have asked my lender to capitalise my overpayment - so it is taken off completely.
There is no specific reason for it - I just like the idea my debt is paying off, rather than hanging around in overpayment reserve.  Also - do not need security of overpayment pillow in case I need to borrow back. (I don't think my mortgage terms  even lets me borrow back).
Just trying to understand what is best practice -   perhaps I do not need to capitalise it on each product transfer and let it grow? Do I gain anything by capitalising overpayment or it is the same if it would still be there in a reserve?
 I am about to go for  another fixed term with the same lender. 

LittleBrownHouse

I wonder what is practice of other borrowers  - so in case you overpay and stay with the same lender for multiple fixed terms - do you capitalise overpayment or let it roll over to next term? Is there any benefit for me by capitalising it?
I must to add that on each new fixed term I reduce repayment term myself. For example my last term was 19 years, with 2 years fixed, now fixed is ending and I am going to take new product with 14 years term.