you should consider a mains water pressure fed electric shower, then the cold water tank being below the shower wont be a problem as it wont be feeding the shower, or if you want to feed the shower with the cold water tank you could try a negative head pumped unit, you will have to look at the manufacturers info to check if they meet your requirements prior to purchase, you can usually download the manuals from the manufactuers websites, if your current shower is a mains fed unit and it is getting its supply from your cold water tank this could be the reason it has never worked, you should check this because if so you may not need to change the shower, just change the feed to it from cold tank to mains feed, hope this helps
H.I.S.
December 2008