As long as the seller puts your UKARA number on a visible place on your parcel you should be fine. Why would customs deliberately seize it and destroy it if you've got a valid defence?
I think the bigger issue usually is that how much more extra you have to pay because of brexit and if it's worth it overall.
Personally I only bought a RIF once from Hong Kong (rainbow8) but it arrived with no issues.
Have a search on the forum, seems like they don't check every rif coming into the country, spot checks. I believe if your your rif gets singled out it gets sent to a different center for examination (Coventry) ??
It's always been unclear how border force get access to ukara, what their process is, whether or not putting your number on the package makes any difference, however can't not help I suppose.
Over the past vcra years I've imported quite a number of RIFs without a ukara number and all have sailed through (lucky for sure).
Expect to pay 20% VAT, 3% Duty and small handling charge (£10) varies.
*** This info is a few years old now and based on my own personal experience.
Only thing you can do is ask them to put your UKARA number on the package.
Then its... ?