VTree's My Golf Game has been on my keep-an-eye-on list for some time. Well it's out and looks very nice indeed. Accessibility features include a range of input methods, including a one-switch mode (game-play only - not for the menus I'm sorry to say) and a range of difficulty levels. There's a promo over at AbleGamers to get a cheaper copy which can save you a few quid.
More golf accessibility? It's possible to play Everybody's Golf for PSone and PS2 in one-switch style (again without fully accessible menus) using a PC, emulator and 4Noah utility. For something much easier to get going, I'd highly recommend trying out the 2D mini-golf accessible classic Mini Golf 1-Button Style by Danjo, which is fully one-switch compatible.
For those thinking of producing their own accessible golf game, I recommend taking a look at the IGDA's GASIG's Top 3 Accessibility Tips for a Golf game.