At Welterweight...he stands a chance against Penn...but I wouldnt take that lightly...if he falls he'll be submitted. British Fighters have no backgrounds in ground fighting...everything for that is learned from the commencement of MMA training, not a specific discipline...this means any ground tricks...and Penn has too many of them, will probably work.
As for Matt Hughes...you know...on the feet Dan Hardy has an advantage...however, if Hughes gets him to the ground he's likely to stay there....and with his strength Matt Hughes would pull off a decision, presuming that Hardy made no errors on the ground...if he makes an error, it is because of Experience as much as skill that Matt Hughes would be able to take advantage and get a submission.
I know it sounds strange...but Dan Hardy is more likely to beat those considered present day contenders...then those who either were contenders, or who are former champions...regardless of whether they are contenders now. Experience counts...and a staunch Wrestler, or good BJJ guy is not going to do Dan Hardy any good....and he has a lot to loose...if he looses vs either because of their style, people will hold it against him as a lack of his skill and he wont get the chance to fight higher tier, with styles that are better matched.
If you are not supremely well rounded, then you need to pick and choose to make it...or put off the stylism clashes until your experience alone can equal well rounded.....Guys like Hughes and Penn are groundsmen primarily...and unlike GSP, they are less afraid to committ and more likely to finish
Now...I would NEVER suggest Hardy Vs Hughes....but Hardy Vs Penn in a year IF Hardy is still excelling, and Penn is tiering with the Division.