To be fair, if you have substantial savings incl max premium bonds, then you should invest, because otherwise, you're at risk of losing a significant portion of your cash wealth due to high inflation. So an IFA would not be doing their job very well if they didn't recommend investing.
Of course, if you have sufficient assets to outlast you, you might not be bothered about them not growing/keeping up with inflation, but otherwise investing is something you should really consider. You're also missing out on significant tax relief by having cash savings, instead of pensions.
It's probably the case that something like Vanguard SIPPs and ISAs are sufficient for most people, but you've eluded to having significant assets, so you could be part of the minority who would benefit from consulting an IFA, even if this is only a one off, rather than ongoing relationship.
You can now work on a fixed fee basis, so they aren't necessarily chasing commissions (not sure if they can even work on commission these days). So it could be that you pay an IFA a few £k, they tell you to maximise your pension contributions, use your ISA allowance, keep enough in cash to last a year or two plus any imminent big purchases and then invest the rest, after having a conversation about your attitude to risk. Then strategically convert the investments to cash to keep an income to live on coming, while reducing the risk of having to draw on it at a bad time. But there's probably other 'advanced' stuff because of your large assets that I don't really know about which is where your IFA will give you the value that you are paying for.