That's not "very little" - that's amazing.
I think the important thing you're missing OP is thinking about what happens next once the mortgage is paid off. The income doesn't disappear - you can build a tidy lump sum in the remaining years to retirement.
When you are approved for a mortgage your income is "stress tested" - they work out what level of repayments would get you into serious financial trouble and they don't lend you that much, because no one knows what interest rates and the economy will do over the next 25 years.
So generally if you are approved for a mortgage of 25 years with a monthly repayment of £800 with an initial 2 year fixed rate, it's because the bank reckons that if interest rates shoot up over the next 2 years, when that 2 year rate is over you'll still be able to afford your mortgage if your repayments are recalculated to £1000pm.
Of course you can spend the "spare" £200pm on a nicer lifestyle now. Or you could put some of it in savings (which may or may not outperform the effect of mortgage overpayments, there's too many variables to know).
But if you choose to put an just £90 extra into the mortgage as a regular overpayment, and actually interest rates stay stable, then instead of paying £240,000 over 25 years you end up paying the mortgage off 4 years early and have paid only £225,000 - but then the crucial thing is that if you then carry on putting aside that £890 per month for the remaining 4 years of the original mortgage term you then have a lump sum of about £45,000 at retirement as well as being mortgage free.
Whereas if instead of putting that £90 into mortgage repayments you just put it into a savings account, you'd still be mortgage free after 25 years but the savings pot (if you chose a risk-free option) would only be about £35,000 - so using mortgage overpayments instead of savings for that has made you £10,000 richer.
You can put the £90 per month into a different investment product which might, if the economy does well, get you even more than £45,000 - or it could and up being only £20,000 if the economy does badly.
Of course you can just spend the £90 on immediate luxuries every month, but if you get used to that level of lifestyle luxury then it hurts a lot more when interest rates do go up, because your lifestyle has to take a hit. Whereas if you are used to overpaying anyway, a hit in interest rates is fine. If monthly repayments go up from £800 to £880 but I am used to paying £890 anyway I can keep my payment static at £890 until such time as I get a pay rise and can choose a new repayment I feel I can afford.