| The standard cans don't sound too bad, but as they are "road" cans, they are heavily silenced.
There are several options on aftermarket cans and a lot of the consideration depends on the £'s you want to spend.
As for the legality, anything without the E mark or BS stamp is illegal.
What you are starting to see now is more and more of the aftermarket stuff coming in stamped up, but with a removable baffle. This is usually held in with a screw which has been tack welded in place.
Crack the screw, remove the baffle and you have a "race" can but with the correct stamps on it. Probably not completely legal, but it may not land you in so much hot water??
These newer cans are more expensive than the normal race cans, but the extra may well be worth it. | 
Threaded Mode