I first want to ask, what do you mean by "UK Rap" because UK Drill/UK hip-hop and grime are two completely different things. If you lump them together and don't know the difference between the two, it probably means you haven't done enough research into either genres.
I encourage you to explore more stuff in both genres, regardless. Surely you haven't heard some truly great works in both scenes and what you've experienced so far is not that impressive to you. For getting into the grime scene, you can start here, for UK rap, I would listen to J Hus, Giggs, Roots Manuva, Nines, Akala, Wretch 32 etc. UK Drill, I would go with Section Boyz, 67, and House of Pharaohs.
Frankly, I think the average UK MC is better than the average American given the rise of trap, which has somehow favoured simplified lyricism and an emphasis on vocal feeling rather than bars. (which is fine and enjoyable, but that doesn't make them a better rapper skillwise.)
The average UK MC can spray bars at a fast pace and, while not deep as Kendrick or anything, usually are nutty, clever bars that are similar in content and meaning to something a 90s NY rapper would spit: lyrics that are boastful, brash and threatening. There is more use of wordplay, and more off the top critical and creative thinking than the average American MC.
But that's not to say they are better than American MCs, the best MCs in America are far better than the best of the UK. The most creative MCs in the States can expand to cover topics such as social inequalities and more nuance, whereas few in the UK can do that.