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Here's a survey of what was being played (in competitions) in the UK last year, ancients rules-wise:

https://madaxemandotcom.blogspot.com/2018/05/whos-playing-what-2018-update.html

The most popular, and fastest growing was ADLG. The UK doesn't have a "Triumph" scene, but it is popular in the US.

As everyone else has rightly said, finding a local group and seeing what they are playing is the best starting point by far

The one near-constant in ancients rules is that basing has pretty much converged on what's known as the "DBx" standard, which for 15mm troops is a 40mm wide base 'element' with differing depths for different troop types. These are broadly speaking 15mm deep for close formation infantry (your pikemen), 20mm for loose formation and skirmishers (peltasts, bowmen and javelinmen), 30mm for mounted troops (Companions and light horse) and 40mm for elephants. Base up like that and you can pretty much play any system out there