The higher octane value is unlikely to make a blind bit of difference unless you have really boosted the engine or are going to zoom up the Alps. In ordinary life, the selling point of these fuels are primarily higher detergent levels than standard fuels, which remove carbon deposits and gunk in the fuel system, giving better combustion, cleaner valves, etc. They may also contain friction modifiers which are claimed to improve engine efficiency, though the lube oil is far more important in this regard. For a car of this age which has never used a premium fuel there may be worthwhile benefits from filling up with a premium fuel for three to six months to clean out the system and see if you can identify improved performance and fuel consumption as a result. After that go back to standard.
As my P300 spends a fair bit of time off the road (tragically) I try to fill up with premium fuel before it is garaged to protect against corrosion in the fuel system and keep the injectors clean; gasoline is very volatile and evaporates quickly, leaving gunk. Otherwise standard fuel is fine from a reputable brand; I’m wary of some of the heavily discounted supermarket fuels as one reason they are cheap is that the additive pack (if any) to the base gasoline is the bare minimum to meet the mandated selling quality. My routine fill is the Costco premium fuel, closely followed by Tesco Momentum, both offering significant savings over the competing retail brands. Costco fuel stations are also the most efficient and easiest to use, they are how all petrol stations should be; won’t happen now for obvious reasons. And Costco hot dogs are the canine cojones at £1.50 (no, I don’t have shares, sadly).
Another option is to try a fuel additive which does much the same job at a much lower price. This is unlikely to increase the fuel octane rating but it does provide the desirable detergents. I would suggest a specialised supplier like Hydra-Fuel rather than the heavily promoted (and very expensive) brands like Wynns and STP. It’s a bit of a faff though, needing funnels and stuff. Easier to just pump it.