Why on earth do you need a third party for your tax calc and resolution? It is surely the antithesis of the "land of the free" and all that stuff.
In the boring old UK I login to a govt. run website to do my annual tax self assessment. My P60 is already filled in (that's my PAYE - pay as you earn, my normal salary) I'm a company director too so I have dividends and expenses and offsets etc to worry about. It takes around an hour to complete.
Most UK employees don't do self assessment. PAYE is such a simple and obvious idea - routine taxation should be done by routine.