I have never heard anyone in British aerobatics use the term 'hammerhead' as an alternative to 'tailslide'. If it was the term used at one time it has long since ceased to be in regular use. Most people hearing the word would assume that a stall turn was meant but the individual had used U.S. terminology.
However I have often heard people refer to the 'hammerhead canopy up (down) out of the tailslide' as a reference to the exit portion of the figure.