I received the update on boxing day on my P20 Pro and my wife received it January 6th on her P20, but my son is still waitng for his update as of tonight for the P20 Pro that he has. What is really weird is that we bought all the phones at the same location on the same day and we live at the same residence. When I have contacted Huawei about the updates discrepancy, they keep saying that the updates are being released in batches based on IMEI and geographic location, so I don't get why my son and I have IMEI numbers that are almost exactly the same and have an identical geographic location, but he is almost a month behind in receiving his update. Next phone I buy will be based strictly on customer service and NOT on camera advertising.
My P20 Pro, on the Telus Network, got its Pie update on Dec 25th. Purchased the P20 new on Feb 20th with updates not coming through until we inserted the SIM, after that we first received UMUI 8.1.xx then EMUI 9.0.0 including the Januay 1st security patch. This even though the Telus Software Update Schedule page isn't listing the P20 as a recipient.
I have the P20 & Pro, here on Telus in Canada, both running EMUI 220.127.116.113 (C792). So if you say you're on 18.104.22.168 (C432) I'd say that's definitely way out of date even for the UK. What month security patch are you on? Mine are both running November 2019.
In regards to unlocked. As of Dec 1st 2017 it was mandated here in Canada all phones after said date must be unlocked yet I'm not convinced, especially in the case of Huawei phones, that has much of a bearing on whether one receives updates or not. Can't comment on the UK because it may be different there but either way you should be on EMUI 9.1.0 (Pie) and not Oreo.