Seriously, best/greatest CURRENT UK sportsmen has to be Andy Murray, as he's currently at the pinnacle of his sport. A couple of years ago it would have to have been Rory McIlroy who was sitting happily as the World Number 1 in his sport.
The 'current' best/greatest will continually change.
How though do you judge it if we have 2 world number ones in 2 different sports? Is one better than the other because he's in one sport rather than another?
It's personal preference.
However, the greatest EVER, that's just opinion.
And even then I think I could only give an opinion PER SPORT