Why are UK pins sealed in HK travel adapters?
A couple months ago, I bought a similar one in the 7-11 at arrivals at HKIA. The "UK" side was sealed- actually some plastic covers that were molded in place. 

The reason, I assume, is that UK-style plugs are used in Hong Kong and the adapter is not approved for use in Hong Kong. 

This is not uncommon- they often don't get approvals for the country in which the adapter is sold, so it's not really approved for use anywhere. It's just more obvious when it's a multi-adapter. 

I can't tell you what to do but I simply broke the little covers off with a pocket knife and proceeded to use it in Hong Kong. I figured this out pretty quickly- the gal was not going to sell it to me if I was going to use it in HK, even with it sealed. Quickly changed from wavering to quite sure that it would never be used in HK. This style (probably like yours) does not provide path a ground so it may be unsafe if a ground pin is necessary for safety. In my case, all the items I had were 2-wire (no ground) except a genuine Dell adapter and I was willing to take a chance on that. 

This one is pretty well made by my professional estimation and I have not died from it yet. Note the lack of a functional ground pin though (it's plastic). Some of the sockets have a shutter that only opens when pushed with the ground pin. Yours looks similar- the ground pin looks to be plastic, and the printed warning confirms that, though 3-pin US/Canada style plugs will fit into it (kind of a no-no- things are expected to be idiot-proof these days).  



I suspect the HK authorities been cracking down on this sort of thing because of poor quality adapters on the market.



Edit: 

Re @jsotola's comment- below is a photo of a Chinese (or US/Canada) 3-pin to 3-pin British adapter, also purchased in HK (years ago, at some shop in Apliu Street). 

It has a proper ground connection that works with either a Chinese/Australian 3-pin plug or a US/Canadian 3-pin plug. Note it is clearly marked "for export only". I guess that's no longer good enough.