I dunno, noone is going to convince me that Jimmy Anderson, Stuart Broad and Graeme Swann are truly great bowlers, but they got their team to number one. Saeed Ajmal is a truly great bowler, and Junaid, Mohammed Irfan and (maybe) a returning Amir would probably be up there with SA as the strongest attack in world cricket.
From what I have seen over last 30-35 years,
Jimmy Anderson - not a great bowler but a very good one; I think credit is due when he delivers in India and Australia as well.
Stuart Broad - a decent bowler in favourable conditions
Graeme Swann - a very good bowler, perhaps one of the best off-spinners of modern era remembering that post-1970s, only Harbhajjan Singh, Saqlain Mushtaq and may be Saeed Ajmal could fall in that category. He has won you games in India (outbowling Indian spinners in India is no joke) and South Africa etc. Very, very underrated. Muralitharan is not an orthodox spinner.
Junaid Khan - not so good.
Mohammad Irfan - made his debut a decade too late. He will end up as a decent bowler at best.
Mohammad Amir - exceptional talent. Don't pin your hopes on his revival though. Five years without cricket at such a young age is not a good sign. Had he and Asif continued, the Pakistanis would have had the most versatile attack in the world.
The point to note here is that not everyone is going to be the leader of the pack. Since you come from New Zealand, I remember Ewen Chatfield of New Zealand who was a very good bowler but only in the support role. Similarly, Junaid Khan can be a very good bowler but to expect him to lead an attack to wins is simply not on. Irfan too can be a very good support bowler. Umar Gul, when he started, was playing second fiddle to Shoaib Akhtar, Mohammad Sami and Shabbir. His record early on was exceptional. Then he got injured and found that Sami was forced to lead a helpless attack with Shabbir's career finished and Shoaib not bowling at all. Suddenly he was asked to lead an attack and his performances went downhill. The same happened with Sami.
The point is that it is for captains to identify the talent, ability and personality of the bowler and ask him to fulfill that particular role.
Imran Khan was very good at it. He would take himself off denying himself easy wickets and instead bring on an embroynic Wasim Akram to get the last wickets so that his confidence could go up. But again, Akram always had it in him to lead attacks. Aqib Javed had it too but was not adequately skilled. Sami had a lot of skill but was always a follower, not a leader.
Coming to the English XI, from what I have seen, Broad has leadership traits but lacks both skill and brain for now. Both of these he can grow in with time. From the 1980s and 1990s, you had Graham Dilley who was basically a good support bowler but was forced to lead the attack because of dearth of fast-bowling talent. He did so manfully for a while but was not meant for that.
I think the smartest English bowler I ever saw was Angus Fraser. He knew his role and limitations and was crystal clear about it. Even when forced to step up, he seemed to let his captains know not to expect too much but would then outdo himself from whatever support role he had to play. To top it, he was a thorough gentleman of a very gentle disposition.