I have a Venture seat bag it zips around and gets taller if needed or shorter if needed and you can stick your helmet in there. i use it all the time i have put some gym stuff in it and you can take large bottles of water in it on long rides. it cost me about $120 Aussie $ thats about 80 pounds. and it is easy to take off its just 4 clips.
Problem with ventura it's a great soft bag system but has the insecurity if any soft bag system. The renntec set up is good but again the rack is just tubular steel, looks ugly as heck.
I also quite like the idea of sw-motech blaze panniers, they're smart as heck, but still soft luggage.