There's not many British rappers I rate to be honest and the ones I do rate all started on Grime anyway, I rate
Mouse Outfit but they're a production team. If it's good it's good but I just don't think the sound goes well with our accents. The vast majority of it is utter turd as well. Not sure who the most famous rapper is from here, maybe Giggs? He is a one trick pony though and really boring. A lot of British rappers try too hard to be 'deep' or intelligent and I can't listen to it, rappers like Akala, Lowkey and so on. Just toss.
Skinnyman is rated very highly but I haven't bothered to check any of his stuff out.
I think either Dot Rotten or Big H is my favourite cos they do what they do well and appeal because they are good, H doesn't force anything and they generally make good music. H's content is really limited but I don't mind that because he has quality control and is unique.
http://www.youtube.com/watch?v=rjrD42VNLNM
http://www.youtube.com/watch?v=U431KEcIp_8
http://www.youtube.com/watch?v=m4SihijGYFw