In the grand scheme of things I'd prefer Police Scotland to concentrate on things such as using the full extent of their powers to make roads safer places for all.
You can pay to have your bike marked if it gives you peace of mind, or you can take details of serial numbers and give them to your insurance company.
If your bike is as expensive or irreplacable as to require special marking, you're likely to be savvy enough to have extra insurance for it and/or stump up to have it marked for less than the cost of a decent lock.
A serial number can be ground off a frame - but what's to stop security markers also being located and removed (particularly when people put a sticker on the bike advertising its presence!). Frankly I'm sceptical about the Bikeregister thing - it doesn't offer any evidence about how your bike is less likely to be stolen if it's marked. I think it's a case of more likley to be recovered and re-united with you. Or is it that people who go to the trouble of getting their bike marked are also more likley to go to the trouble of trying to recover it? I certainly don't see that they've tested that hypothesis amongst the website blurb. £11 for a sticker for your bike frame sounds like a bit of a get-rich-qucik scam to me.