I can help you out with some cheaper rates on those, around 29-30rmb/kg at 100+kg with UPS to the UK. My email is firstname.lastname@example.org if you have any shipments coming up I can quote you on them and see how it stacks up.
Be very, very careful who you use. Best to stick to a reputable carrier like you have now really. It is a minefield in China, especially 'agents'
Though it's impossible to go direct to the carrier in China and get a good rate with intl express couriers. Unless you have very large volume, agents are a must in order to allow consolidation on their account and bring the price down to something economical, it's just like factory selection - some are more reliable than others.
Most of the Shenzhen courier forwarders/agents use their Hong Kong accounts, and truck your goods there first for better rates as the Shenzhen depots of the big global couriers give higher prices per kg. If you understand the process and ask the right questions, the good agents do add value.