For a US based single engine aircraft, figure US$14,000 for an S-tec two axis model, installed.
Fairly straight forward procedure, but the complications begin if you desire the autopilot to be equipped for GPS auto-tracking, approaches, etc.
A few extra bits required, as well as additional labor, of course.
Dunno about the prices in the UK, but would expect the whole shebang to cost about a third more there.
As you probably already know, prices a cheaper in the USA.
Regulatory requirements less as well.
Hpoe this helps.