Ok, bear with me here. I'm going to throw a lot of figures at you but I've bolded the key sections: Your annual spending is £19,200, but in five years you will receive the state pension. Let's presume you earn the average state pension (£153.36 per week) you can subtract that figure from your annual spend (£19,200 - £7,974.72) which gives you £11,225.28 per year, that you'll need to withdraw from pensions annually.
The universal rule with withdrawing from pensions is called 'the 4% rule'. There's lots of information about it online. It presumes: your pension will grow by an average of 7% pa and that inflation will rise by an average of 3% pa. Therefore you can withdraw 4% of the total value of your pension per year and it should last as long as you do.
So to find out what you need in your pension to retire (in the simplest form) is to multiply £11,225.28 by 25 which gives you £280,632. (280,632 x 0.04 = £11,225.28). Around £280k is the figure you need invested when you reach state pension age to retire on £19.2k pa.
Now if you want to retire or go part-time prior to state pension age, you should really make up the deficit so you don't risk financial insecurity in later life. If your part-time covered the state pension (around £8k pa) that would suffice. To fully retire you would need 25x your annual spending at state pension age + the equivalent of 5 years worth of state pension (£39,873.60) which is £320,505.60 invested sensibly today.
If your total pension contributions are £125k, when you reach state pension age you can withdraw £5k safely pa and your state pension of roughly £8k means you'll receive around £13k pa or just over £1k per month.
Once you've considered this, then you can really start to consider where you are with work and if going part-time or retiring is the right decision.