The term 'best cuber' is rather subjective as the World Cube Association features many different events, and different cubers specialise in different events. The most popular event is undoubtedly the original 3x3x3 and the current national record holder for 3x3x3 average is James Alonso. For 3x3x3 single, Chris Mills is 0.01 seconds ahead of James Alonso but cubers usually focus on consistency of the average. Both Chris Mills and James Alonso have 10 of the top 20 3x3x3 averages in the UK.
However, looking at other events, Chris Mills and James Alonso certainly aren't bad at them, but other cubers are faster, Luke Burns is well ahead of everyone else in the UK at 2x2x2, Daniel Partridge is well ahead at pyraminx, Fabio Schwandt's skewb NR average still stands from 2018, Nicholas Archer is now well ahead at 3x3x3 one-handed, winning some competitions with an average almost half that of second place. 3x3x3 blindfolded is very competitive in the UK with Ben Stokes narrowly hanging onto the national record, but with close competition from Jaye Sloan and Charlie Harisson.
Looking at sum of ranks, Daniel Partridge is now in the lead, he is now in the top 5 for 7 different events and continually seems to improve at them all. Fabio Schwandt is still up there but seems to have accepted that Daniel Partridge is now overall the best cuber in the UK. We will have to wait and see how long this lasts as many new cubers keep coming up and being very competitive.