You can't say this! Or is any athlete who improves significantly certain to be using PEDs? If that's the case, why should any athlete train hard in an attempt to get better - they won't get any credit for it and will be accused of cheating their way to the top!
The athletes named in this thread are all different cases. Muir was world class last year. OK, she has run quicker this year but it's not as if she was running 4:10 last year.
Surely, we can all cite cases of friends/training partners who have made very significant leaps forward in the course of twelve months? This can be due simply to doing more training, less training (in a surprising number of cases) or simply different training - switching from a regime based on high volume to a more quality based approach (or visa versa).
I'm not saying none of these athletes could possibly cheat because they are Brits but if they, or anyone else, are going to be accused of cheating there has to be a bit more evidence behind the claims than "they've got a lot quicker"!