Yes is the short answer - there are several makes and this is but one.
http://www.mirashowers.com/onlinecatalog/acces sory_features.jsp?item=339316&prod_num=Mira+En duro+Universal+Pump&cat=Pump
You will note that this is dual pump. That is the way they all work - you should be pumping up the pressure on both hot and cold feeds. This maintains them in equal balance.
But there is another fundamental flaw with your existing installation - your cold feed is taken off the pressurised mains supply yet your hot feed comes from the pressure created by the head of water to the loft tank. It shouldn't be like that - the cold feed should come from the same source - bypassing the hot water cylinder of course - this maintains the input pressure to both sides in balance (albeit lower than mains pressure).
If you modify the feeds as per the above, the shower will then work in balance for temperature. At that point you may find the volume of water dispensed is inadequate and at that point you could fit the pump. BM