Is "Tally-Ho" used in ATC radio communication, specifically in the UK?
I have never heard "tally ho" used in civilian aviation and it not a recognised phrase so should not be used.  A civilian ATCO would not think positively about anyone using that phrase.

It used to be used in military comms in combat.  Its usage arose during WW1 when the Royal Flying Corps (and later the Royal Air Force) drew its crews mostly from the "officer classes" who, in general, were also fox hunters or supporters of the same.

"Tally ho" is the cry shouted out by a huntsman when the fox is spotted and the hunt is on.  

I served in the RAF from 1976 to 1986. Then as a civilian in the military until 1993 and in that time never heard the phrase.  I suspect that it has died out since it is not standard phraseology and perpetuates stereotypes which have no place in modern, professional organisations.

A non-standard, but common phrase is a simple "visual", but the correct phrase in both civilian and military use is "traffic in sight".