I am considering changing the valves on the shower supply pipes to
gate valves (the ones with the hand wheel).

Don't ever install a gate valve. They are appalling designs. If you are
worried about flow, then use full bore quarter turn lever valves.

http://www.screwfix.com/app/sfd/cat/...36042&id=13103
http://www.screwfix.com/app/sfd/cat/...36042&id=17293

Christian.