I'm not exactly sure which model you have, so you need to double check for yourself. The handbook will give the plug reference.
If it's the same as my 2018, then a Denso equivalent plug will be around £20 from a reputable supplier. I usually get plugs etc from either
https://www.sparkplugs.co.uk/ or
https://www.gsparkplug.com/Put in your registration to avoid errors.
Denso tend to be a bit cheaper than NGK, I'm happy to use either whichever works out cheaper.
Beware there are plenty of fake spark plugs around from auction site sellers, as you have noted they are expensive items so lots of potential money to be made by scammers. I always use reputable suppliers.
Assuming you have iridium plugs in there, they are good for at least 50k miles so regardless of how old the car is or what the service schedule says, if they are below that sort of mileage and there are no running issues then leave them in. I have run iridium plugs well past 50k miles with no issues.
