Based on some quick Googling, I don't see what is so much better about an SIPP or a group personal pension plan? SIPP offers some kind of matching from the government, but it doesn't seem significant.
For a higher rate taxpayer you put in £60 and get £100 in your pension.
You can also do salary sacrifice and reduce you income tax and NI lability
It's quite likely that some people today will end up paying higher income tax rates in retirement than today.
And you can use drawdown to manage your income from the pension and you would of course make use of income from your ISA.
SIPPs are beyond my lifestyle.