You could take the service for 1 or 2 years and see how it goes. Octopus Cosy heat pump has not been out that long, so not a long history. You can always cancel it and take the warranty risk if the evidence shows that they are extremely reliable. Most electro-mechanical devices will develop faults within 2 years if there is a manufacturing flaw or a bad design issue comes to light, This might be fan/compressor noise, premature bearing failure etc. So minimum of 1-2 years of service/warranty cover a good idea. Warranty also offers peace of mind which is worthwhile, like home insurance.
Most heat pump services do very little, but you are paying for someone’s time. Cleaning debris from inside/outside unit, Cleaning filters, checking water pressure. Unlike gas boilers there is no safety issue to be worried about.
Previous to our Vaillant Arotherm+ we had 2 Vaillant gas boilers which operated without a single problem over 25 years. Our last boiler was a special offer to upgrade from non-condensing to condensing, not because the previous one was broken.
Our Mitsubishi AC unit has been working flawlessly for 8 years and hasn’t had a single service yet.
Manufacturer and history of reliability something to look out for,
If you ever have to replace your HP there is no issue changing make/model. Most folks will only ever need to replace the outdoor unit if it ever wears out which will be considerably cheaper than the whole system and will possibly be even cheaper than today if heat pumps ever reach mass use.