Hi, we're in a similar position with our old people carrier and did a look back at our usage and then looked at the costs for car clubs, taxis and car hire costs if we got rid of it.
In our case the away trips to family, especially during school holidays, makes it cheaper to keep ours and pay ULEZ (the away trips is the big cost for hiring). We rarely use it for local trips and plan usage to aim for multiple purpose trips. We also share with our neighbours such as moving large items with our people carrier and share the ulez cost.
Generally for low use alone it will not make financial sense to buy a newer car.
Also, if you have a petrol car check its NOx output amount for ULEZ compliance as TFL wrongly categorised many older cars. Did this for neighbours 2001 car and it's ULEZ compliant when TFL originally said it was not (now fixed) and having the same issue with a 1997 petrol car.