Thats what I did. My complete shopping list was as follows: mechanical flange
Immersion heater that would fit through the mechanical flange (tried some for size in a local plumbing merchants)
1.5mm square heat resistant cable
A good quality 13a plug
a Ceramic connector block
(thats the type I wanted anyway - i ended up cutting down a bigger one because the 1-ways were out of stock)
I already had a good quality wall socket, and you may not need the connector block if your thermostat happens to have a big enough hole to join the two wires instead.
EDIT: here's a pic of my connector block fitted: