I'm not sure that's true. My mortgage has always quoted a fixed repayment amount for the entire duration. Obviously that changes and gets recalculated whenever anything changes (like lump sum overpayments and interest rate changes).
So not a repayment that reduces each month/yr. That's why in the early part of the mortgage your payments are predominantly covering the interest, and later in the mortgage that same monthly amount is reducing the balance more.
Obviously the interest is compound, so small rate increases make a huge difference over the lifetime of the mortgage. And the more you overpay, and the earlier you over pay it, the biggest the saving long term. It's a bit like pensions in reverse - the earlier you start and the more you pay in earlier, the bigger the advantage in the long term, due to the same compounding effect.
In regards to overpayment allowances, they're usually calculated annually, not on a monthly or daily basis. Typically, at the start of a 12 month period the amount is set for that year based on the mortgage balance at the start of it. You'd have to check the details or your mortgage to find out when that 12 month period runs from/to.
Mine was 30th September, so I managed to use up all of last years overpayment allowance just before the end of the year, and now it's reset for the next 12 months. In theory, I could pay off all my 10% allowance for this next year right now, or I could do it in equal monthly amounts, or I could wait until 29th September 2023 and do it all in one go then.
At the moment, it's possible to get savings rates that are outpaying mortgage interest (if you've got an existing fix at a good rate). E.g. my mortgage rate is 2.4% for another 15 months, but Barclays have just launched a savings account that's paying 5.1% interest. So for now, I'm better off (only by a £100 or so) putting the money into savings than overpaying the mortgage.