you need to select a 'deep water' variety. i would wait until spring now. buy a lily that is specifically for that depth. they need to be in a waterplant basket with a low feed compost. then lower the pot in place while the lily is still dormant/or just showing signs of life. do not have it near a fountain or waterfall as they prefer still waters.

I notice you're in Gloucestershire. If you just pop over into Herefordshire there's Kenchester Water Gardens, which claim to hold the national collection of water lillies. I bought my water lilly from there and the staff are very helpful and will help you choose the right sized water lilly for your pond. If you want to visit them, it's half way between Hereford and Leominster on the A49.

Note that the more of the pond surface is covered with lilies, the less oxygen the fish will have. Water lilies don't release oxygen into the water, the pads keep the surface from giving up CO2, and absorbing oxygen, and they shade any algae or submerged plants that would otherwise use the sun's energy to oxygenate the water.